Golf ball

ABSTRACT

A golf ball  2  includes a core  4 , an inner cover  6 , and an outer cover  8 . The core  4  includes a center  10 , an inner envelope layer  12 , and an outer envelope layer  14 . The inner envelope layer  12  is formed by a rubber composition being crosslinked. The rubber composition includes: a base rubber (a); a co-crosslinking agent (b); a crosslinking initiator (c); and an acid and/or a salt (d). The co-crosslinking agent (b) is:
         (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or   (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

This application claims priority on Patent Application No. 2012-123720 filed in JAPAN on May 30, 2012. The entire contents of this Japanese Patent Application are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls.

Specifically, the present invention relates to golf balls that include a center, an inner envelope layer, an outer envelope layer, and a cover.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance. In particular, golf players place importance on flight performance upon a shot with a driver. Flight performance correlates with the resilience performance of a golf ball. When a golf ball having excellent resilience performance is hit, the golf ball flies at a high speed, thereby achieving a large flight distance.

Golf players also place importance on spin performance of golf balls. When a backspin rate is high, the run is short. It is easy for golf players to cause a golf ball, to which backspin is easily provided, to stop at a target point. When a sidespin rate is high, the golf ball tends to curve. It is easy for golf players to intentionally cause a golf ball, to which sidespin is easily provided, to curve. A golf ball to which spin is easily provided has excellent controllability. In particular, advanced golf players place importance on controllability upon a shot with a short iron.

Golf balls that include a core having excellent resilience performance are disclosed in JP61-37178, JP2008-212681 (US2008/0214324), JP2008-523952 (US2006/0135287 and US2007/0173607), and JP2009-119256 (US2009/0124757).

The core disclosed in JP61-37178 is obtained from a rubber composition that includes a co-crosslinking agent and a crosslinking activator. This publication discloses palmitic acid, stearic acid, and myristic acid as the crosslinking activator.

The core disclosed in JP2008-212681 is obtained from a rubber composition that includes an organic peroxide, a metal salt of an α,β-unsaturated carboxylic acid, and a copper salt of a fatty acid.

The core disclosed in JP2008-523952 is obtained from a rubber composition that includes a metal salt of an unsaturated monocarboxylic acid, a free radical initiator, and a non-conjugated diene monomer.

The core disclosed in JP2009-119256 is obtained from a rubber composition that includes a polybutadiene whose vinyl content is equal to or less than 2%, whose cis 1,4-bond content is equal to or greater than 80%, and which has an active end modified with an alkoxysilane compound.

An appropriate trajectory height is required in order to achieve a large flight distance. A trajectory height depends on a spin rate and a launch angle. With a golf ball that achieves a high trajectory by a high spin rate, a flight distance is insufficient. With a golf ball that achieves a high trajectory by a high launch angle, a large flight distance is obtained. Use of an outer-hard/inner-soft structure in a golf ball can achieve a low spin rate and a high launch angle. Modifications regarding a hardness distribution of a core are disclosed in JP6-154357 (U.S. Pat. No. 5,403,010), JP2008-194471 (U.S. Pat. No. 7,344,455, US2008/0194358, US2008/0194359, and US2008/0214325), and JP2008-194473 (US2008/0194357 and US2008/0312008).

In the core disclosed in JP6-154357, a JIS-C hardness H1 at the central point of the core is 58 to 73, a JIS-C hardness H2 in a region that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point is equal to or greater than 65 but equal to or less than 75, a JIS-C hardness H3 at a point located at a distance of 15 mm from the central point is equal to or greater than 74 but equal to or less than 82, and a JIS-C hardness H4 at the surface of the core is equal to or greater than 76 but equal to or less than 84. The hardness H2 is greater than the hardness H1, the hardness H3 is greater than the hardness H2, and the hardness H4 is equal to or greater than the hardness H3.

In the core disclosed in JP2008-194471, a Shore D hardness at the central point of the core is equal to or greater than 30 but equal to or less than 48, a Shore D hardness at a point located at a distance of 4 mm from the central point is equal to or greater than 34 but equal to or less than 52, a Shore D hardness at a point located at a distance of 8 mm from the central point is equal to or greater than 40 but equal to or less than 58, a Shore D hardness at a point located at a distance of 12 mm from the central point is equal to or greater than 43 but equal to or less than 61, a Shore D hardness in a region that extends over a distance range from equal to or greater than 2 mm to equal to or less than 3 mm from the surface of the core is equal to or greater than 36 but equal to or less than 54, and a Shore D hardness at the surface is equal to or greater than 41 but equal to or less than 59.

In the core disclosed in JP2008-194473, a Shore D hardness at the central point of the core is equal to or greater than 25 but equal to or less than 45, a Shore D hardness in a region that extends over a distance range from equal to or greater than 5 mm to equal to or less than 10 mm from the central point is equal to or greater than 39 but equal to or less than 58, a Shore D hardness at a point located at a distance of 15 mm from the central point is equal to or greater than 36 but equal to or less than 55, and a Shore D hardness at the surface of the core is equal to or greater than 55 but equal to or less than 75.

JP2010-253268 (US2010/0273575) discloses a golf ball that includes a core, an inner envelope layer, an inner cover, and an outer cover. In the core, the hardness gradually increases from the central point of the core to the surface of the core. The difference between a JIS-C hardness at the surface and a JIS-C hardness at the central point is equal to or greater than 15. The hardness of the outer cover is greater than the hardness of the inner cover, and the hardness of the inner cover is greater than the hardness of the inner envelope layer.

For a second shot on a par-four hole and a par-five hole, a fairway wood is frequently used. Golf players desire excellent flight performance upon a shot with a fairway wood. Further, golf players desire excellent feel at impact upon a shot with a fairway wood. An object of the present invention is to provide a golf ball that has excellent flight performance and excellent feel at impact upon a shot with a fairway wood.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a core and a cover positioned outside the core. The core includes a center, an inner envelope layer positioned outside the center, and an outer envelope layer positioned outside the inner envelope layer. The cover includes one or more layers. At least one of the center, the inner envelope layer, and the outer envelope layer is formed from a rubber composition that includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt. The co-crosslinking agent (b) is:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A component having the greatest hardness in a range from a central point of the golf ball to a surface of the golf ball is an outermost layer of the cover. In the golf ball according to the present invention, a hardness distribution of the core is appropriate. In the golf ball, the energy loss is low in the core when the golf ball is hit with a fairway wood. In the core, spin is suppressed when the golf ball is hit with a fairway wood. In the golf ball, the outer cover can contribute to resilience performance. In the golf ball, a large flight distance is achieved upon a shot with a fairway wood. An impact shock is small when the golf ball is hit with a fairway wood. The golf ball also has excellent feel at impact.

Preferably, the cover includes an inner cover and an outer cover positioned outside the inner cover.

Preferably, a difference (Ho−Hs) between a JIS-C hardness Ho of the outer cover and a JIS-C hardness Hs at a surface of the core is equal to or greater than 5. Preferably, a difference (Ho−Hi) between a JIS-C hardness Ho of the outer cover and a JIS-C hardness Hi of the inner cover is equal to or greater than 3.

Preferably, an amount of the acid and/or the salt (d) is equal to or greater than 1.0 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a).

Preferably, the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1). Preferably, a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than 30. Preferably, the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof. Preferably, the carboxylic acid and/or the salt thereof (d1) is a zinc salt of a carboxylic acid. Preferable examples of the zinc salt of the carboxylic acid include zinc octoate, zinc laurate, zinc myristate, and zinc stearate.

Preferably, the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a). Preferably, the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).

Preferably, a difference (Hs−H(0.0)) between a JIS-C hardness Hs at a surface of the core and a JIS-C hardness H(0.0) of a central point of the core is equal to or greater than 15.

Preferably, a sum (Ti+To) of a thickness Ti of an inner cover and a thickness To of an outer cover is equal to or less than 2.5 mm.

Preferably, the rubber composition further includes an organic sulfur compound (e). Preferable examples of the organic sulfur compound (e) include thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof. Preferably, the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).

Preferably, when the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition further includes a metal compound (f).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ball according to one embodiment of the present invention; and

FIG. 2 is a line graph showing a hardness distribution of an inner envelope layer of the golf ball in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based on preferred embodiments with reference to the accompanying drawings.

A golf ball 2 shown in FIG. 1 includes a spherical core 4, an inner cover 6 positioned outside the core 4, and an outer cover 8 positioned outside the inner cover 6. The core 4 includes a spherical center 10, an inner envelope layer 12 positioned outside the center 10, and an outer envelope layer 14 positioned outside the inner envelope layer 12. The outer cover 8 is an outermost layer of the cover. On the surface of the outer cover 8, a large number of dimples 14 are formed. Of the surface of the golf ball 2, a part other than the dimples 14 is a land 16. The golf ball 2 includes a paint layer and a mark layer on the external side of the outer cover 8, but these layers are not shown in the drawing.

The golf ball 2 preferably has a diameter of 40 mm or greater but 45 mm or less. From the standpoint of conformity to the rules established by the United States Golf Association (USGA), the diameter is particularly preferably equal to or greater than 42.67 mm. In light of suppression of air resistance, the diameter is more preferably equal to or less than 44 mm and particularly preferably equal to or less than 42.80 mm. The golf ball 2 preferably has a weight of 40 g or greater but 50 g or less. In light of attainment of great inertia, the weight is more preferably equal to or greater than 44 g and particularly preferably equal to or greater than 45.00 g. From the standpoint of conformity to the rules established by the USGA, the weight is particularly preferably equal to or less than 45.93 g.

In the present invention, JIS-C hardnesses can be measured at 15 measuring points from the central point of the core 4 to the surface of the core 4. The distances from the central point of the core 4 to these measuring points are as follows.

First point: 0.0 mm

Second point: 2.5 mm

Third point: 5.0 mm

Fourth point: 7.0 mm

Fifth point: 7.5 mm

Sixth point: 8.0 mm

Seventh point: 9.5 mm

Eighth point: 10.0 mm

Ninth point: 10.5 mm

Tenth point: 12.5 mm

Eleventh point: 14.5 mm

Twelfth point: 15.0 mm

Thirteenth point: 15.5 mm

Fourteenth point: 17.5 mm

Fifteenth point: surface

Hardnesses at the first to fourteenth points are measured by pressing a JIS-C type hardness scale against a cut plane of the core 4 that has been cut into two halves. A hardness at the fifteenth point is measured by pressing the JIS-C type hardness scale against the surface of the spherical core 4. For the measurement, an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.), to which this hardness scale is mounted, is used.

FIG. 2 is a line graph showing a hardness distribution of the inner envelope layer 12 of the golf ball 2 in FIG. 1. The horizontal axis of the graph indicates distances (mm) from the central point of the core 4. The vertical axis of the graph indicates a JIS-C hardness. In the graph, the four points of the sixth point, the eighth point, the tenth point, and the eleventh point are plotted.

FIG. 2 also shows a linear approximation curve obtained by a least-square method on the basis of the distances and the hardnesses of the four measuring points. In FIG. 2, the broken line does not greatly deviate from the linear approximation curve. In other words, the broken line has a shape close to the linear approximation curve. In the inner envelope layer 12, the hardness linearly increases from its inside toward its outside. When the golf ball 2 is hit with a fairway wood, the energy loss is low in the inner envelope layer 12. The golf ball 2 has excellent resilience performance. When the golf ball 2 is hit with a fairway wood, the flight distance is large.

R² of the linear approximation curve for the inner envelope layer 12 which is obtained by the least-square method is preferably equal to or greater than 0.95. R² is an index indicating the linearity of the broken line. For the inner envelope layer 12 for which R² is equal to or greater than 0.95, the shape of the broken line of the hardness distribution is close to a straight line. The golf ball 2 that includes the inner envelope layer 12 for which R² is equal to or greater than 0.95 has excellent resilience performance. R² is more preferably equal to or greater than 0.96 and particularly preferably equal to or greater than 0.97. R² is calculated by squaring a correlation coefficient R. The correlation coefficient R is calculated by dividing the covariance of the distance (mm) from the central point and the hardness (JIS-C) by the standard deviation of the distance (mm) from the central point and the standard deviation of the hardness (JIS-C).

In light of suppression of spin, the gradient a of the linear approximation curve is preferably equal to or greater than 0.30, more preferably equal to or greater than 0.33, and particularly preferably equal to or greater than 0.35.

In the present invention, a JIS-C hardness at a measuring point whose distance from the central point of the core 4 is x (mm) is represented by H(x). The hardness at the central point of the core 4 is represented by H (0.0). In the present invention, the JIS-C hardness at the surface of the core 4 is represented by Hs. The difference (Hs−H(0.0)) between the surface hardness Hs and the central hardness H(0.0) is preferably equal to or greater than 15. The core 4 in which the difference (Hs−H(0.0)) is equal to or greater than 15 has an outer-hard/inner-soft structure. When the golf ball 2 is hit with a driver, the recoil (torsional return) in the core 4 is great, and thus spin is suppressed. The core 4 contributes to the flight performance of the golf ball 2. In light of flight performance, the difference (Hs−H(0.0)) is more preferably equal to or greater than 23 and particularly preferably equal to or greater than 24. From the standpoint that the core 4 can easily be formed, the difference (Hs−H(0.0)) is preferably equal to or less than 50. In the core 4, the hardness gradually increases from its central point toward its surface.

The center 10 is formed by crosslinking a rubber composition. Examples of base rubbers for use in the rubber composition include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. Two or more rubbers may be used in combination. In light of resilience performance, polybutadienes are preferred, and high-cis polybutadienes are particularly preferred.

Preferably, the rubber composition of the center 10 includes a co-crosslinking agent. Examples of preferable co-crosslinking agents in light of resilience performance include zinc acrylate, magnesium acrylate, zinc methacrylate, and magnesiummethacrylate. Preferably, the rubber composition includes an organic peroxide together with a co-crosslinking agent. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Preferably, the rubber composition includes a sulfur compound.

According to need, various additives such as a filler, sulfur, a vulcanization accelerator, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, and the like are included in the rubber composition of the center 10 in an adequate amount. Synthetic resin powder or crosslinked rubber powder may also be included in the rubber composition.

In the present embodiment, the center 10 is more flexible than the inner envelope layer 12. The center 10 can suppress spin. The center 10 preferably has a diameter of 10 mm or greater but 20 mm or less. In the golf ball 2 that includes the center 10 having a diameter of 10 mm or greater, spin can be suppressed. In this respect, the diameter is more preferably equal to or greater than 12 mm and particularly preferably equal to or greater than 14 mm. The golf ball 2 that includes the center 10 having a diameter of 20 mm or less has excellent resilience performance although the center 10 is flexible. In this respect, the diameter is more preferably equal to or less than 18 mm and particularly preferably equal to or less than 16 mm.

The inner envelope layer 12 is formed by crosslinking a rubber composition. The rubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

Examples of the base rubber (a) include polybutadienes, polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-diene copolymers, and natural rubbers. In light of resilience performance, polybutadienes are preferred. When a polybutadiene and another rubber are used in combination, it is preferred that the polybutadiene is included as a principal component. Specifically, the proportion of the polybutadiene to the entire base rubber is preferably equal to or greater than 50% by weight and more preferably equal to or greater than 80% by weight. The proportion of cis-1,4 bonds in the polybutadiene is preferably equal to or greater than 40% by weight and more preferably equal to or greater than 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal to or less than 2.0% by weight is preferred. The polybutadiene can contribute to the resilience performance of the golf ball 2. In this respect, the proportion of 1,2-vinyl bonds is preferably equal to or less than 1.7% by weight and particularly preferably equal to or less than 1.5% by weight.

From the standpoint that a polybutadiene having a low proportion of 1,2-vinyl bonds and excellent polymerization activity is obtained, a polybutadiene synthesized with a rare-earth-element-containing catalyst is preferred. In particular, a polybutadiene synthesized with a catalyst containing neodymium, which is a lanthanum-series rare earth element compound, is preferred.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) of preferably 30 or greater, more preferably 32 or greater, and particularly preferably 35 or greater. The Mooney viscosity (ML₁₊₄(100° C.)) is preferably equal to or less than 140, more preferably equal to or less than 120, even more preferably equal to or less than 100, and particularly preferably equal to or less than 80. The Mooney viscosity (ML₁₊₄(100° C.)) is measured according to the standards of “JIS K6300”. The measurement conditions are as follows.

Rotor: L rotor

Preheating time: 1 minute

Rotating time of rotor: 4 minutes

Temperature: 100° C.

In light of workability, the polybutadiene has a molecular weight distribution (Mw/Mn) of preferably 2.0 or greater, more preferably 2.2 or greater, even more preferably 2.4 or greater, and particularly preferably 2.6 or greater. In light of resilience performance, the molecular weight distribution (Mw/Mn) is preferably equal to or less than 6.0, more preferably equal to or less than 5.0, even more preferably equal to or less than 4.0, and particularly preferably equal to or less than 3.4. The molecular weight distribution (Mw/Mn) is calculated by dividing the weight average molecular weight Mw by the number average molecular weight Mn.

The molecular weight distribution is measured by gel permeation chromatography (“HLC-8120GPC” manufactured by Tosoh Corporation). The measurement conditions are as follows.

Detector: differential refractometer

Column: GMHHXL (manufactured by Tosoh Corporation)

Column temperature: 40° C.

Mobile phase: tetrahydrofuran

The molecular weight distribution is calculated as a value obtained by conversion using polystyrene standard.

Preferable examples of the co-crosslinking agent (b) include:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms.

The rubber composition may include only the α,β-unsaturated carboxylic acid (b1) or only the metal salt (b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent (b). The rubber composition may include both the α,β-unsaturated carboxylic acid (b1) and the metal salt (b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent (b).

The metal salt (b2) of the α,β-unsaturated carboxylic acid graft-polymerizes with the molecular chain of the base rubber, thereby crosslinking the rubber molecules. When the rubber composition includes the α,β-unsaturated carboxylic acid (b1), the rubber composition preferably further includes a metal compound (f). The metal compound (f) reacts with the α,β-unsaturated carboxylic acid (b1) in the rubber composition. A salt obtained by this reaction graft-polymerizes with the molecular chain of the base rubber.

Examples of the metal compound (f) include metal hydroxides such as magnesium hydroxide, zinc hydroxide, calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide; metal oxides such as magnesium oxide, calcium oxide, zinc oxide, and copper oxide; and metal carbonates such as magnesium carbonate, zinc carbonate, calcium carbonate, sodium carbonate, lithium carbonate, and potassium carbonate. A compound that includes a bivalent metal is preferred. The compound that includes the bivalent metal reacts with the co-crosslinking agent (b) to form metal crosslinks. The metal compound (f) is particularly preferably a zinc compound. Two or more metal compounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examples of the metal component in the metal salt (b2) of the α,β-unsaturated carboxylic acid include sodium ion, potassium ion, lithium ion, magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminum ion, tin ion, and zirconium ion. The metal salt (b2) of the α,β-unsaturated carboxylic acid may include two or more types of ions. From the standpoint that metal crosslinks are likely to occur between the rubber molecules, bivalent metal ions such as magnesium ion, calcium ion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt (b2) of the α,β-unsaturated carboxylic acid is particularly preferably zinc acrylate.

In light of resilience performance of the golf ball 2, the amount of the co-crosslinking agent (b) is preferably equal to or greater than 15 parts by weight and particularly preferably equal to or greater than 20 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact, the amount is preferably equal to or less than 50 parts by weight, more preferably equal to or less than 45 parts by weight, and particularly preferably equal to or less than 40 parts by weight, per 100 parts by weight of the base rubber.

The crosslinking initiator (c) is preferably an organic peroxide. The organic peroxide contributes to the resilience performance of the golf ball 2. Examples of preferable organic peroxides include dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. In light of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of the crosslinking initiator (c) is preferably equal to or greater than 0.2 parts by weight and particularly preferably equal to or greater than 0.5 parts by weight, per 100 parts by weight of the base rubber. In light of feel at impact and durability of the golf ball 2, the amount is preferably equal to or less than 5.0 parts by weight and particularly preferably equal to or less than 2.5 parts by weight, per 100 parts by weight of the base rubber.

In the present invention, the co-crosslinking agent (b) is not included in the concept of the acid and/or the salt (d). It is inferred that the acid and/or the salt (d) breaks the metal crosslinks by the co-crosslinking agent (b) in the vicinity of the inner surface of the inner envelope layer 12 during heating and forming of the core 4. Examples of the acid and/or the salt (d) include oxo acids such as carboxylic acids, sulfonic acids and phosphoric acid, and salts thereof; and hydroacids such as hydrochloric acid and hydrofluoric acid, and salts thereof. Oxo acids and salts thereof are preferred. A carboxylic acid and/or a salt thereof (d1) is more preferred. Carboxylates are particularly preferred.

The carbon number of the carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is preferably equal to or greater than 1 but equal to or less than 30, more preferably equal to or greater than 3 but equal to or less than 30, and even more preferably equal to or greater than 5 but equal to or less than 28. Examples of the carboxylic acid include aliphatic carboxylic acids (fatty acids) and aromatic carboxylic acids. A fatty acid and a salt thereof are preferred.

The rubber composition may include a saturated fatty acid and/or a salt thereof, or may include an unsaturated fatty acid and/or a salt thereof. The saturated fatty acid and/or the salt thereof are preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5), caproic acid (C6), enanthic acid (C7), caprylic acid (C8), pelargonic acid (C9), capric acid (decanoic acid) (C10), lauric acid (C12), myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15), palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17), stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid (C18), linolic acid (C18), linolenic acid (C18), 12-hydroxystearic acid (C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid (C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22), lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanic acid (C28), and melissic acid (C30). Two or more fatty acid salts may be used in combination.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group. Examples of aromatic carboxylic acids include benzoic acid, phthalic acid, isophthalic acid, terephthalic acid, hemimellitic acid (benzene-1,2,3-tricarboxylic acid), trimellitic acid (benzene-1,2,4-tricarboxylic acid), trimesic acid (benzene-1,3,5-tricarboxylic acid), mellophanic acid (benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid (benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid (benzene-1,2,4,5-tetracarboxylic acid), mellitic acid (benzene hexacarboxylic acid), diphenic acid (biphenyl-2,2′-dicarboxylic acid), toluic acid (methylbenzoic acid), xylic acid, prehnitylic acid (2,3,4-trimethylbenzoic acid), γ-isodurylic acid (2,3,5-trimethylbenzoic acid), durylic acid (2,4,5-trimethylbenzoic acid), β-isodurylic acid (2,4,6-trimethylbenzoic acid), α-isodurylic acid (3,4,5-trimethylbenzoic acid), cuminic acid (4-isopropylbenzoic acid), uvitic acid (5-methylisophthalic acid), α-toluic acid (phenylacetic acid), hydratropic acid (2-phenylpropanoic acid), and hydrocinnamic acid (3-phenylpropanoic acid).

The rubber composition may include an aromatic carboxylate substituted with a hydroxyl group, an alkoxy group, or an oxo group. Examples of this carboxylic acid can include salicylic acid (2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinic acid (hydroxy(methyl) benzoic acid), o-homosalicylic acid (2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid (2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid (2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid (2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoic acid), γ-resorcylic acid (2,6-dihydroxybenzoic acid), protocatechuic acid (3,4-dihydroxybenzoic acid), α-resorcylic acid (3,5-dihydroxybenzoic acid), vanillic acid (4-hydroxy-3-methoxybenzoic acid), isovanillic acid (3-hydroxy-4-methoxybenzoic acid), veratric acid (3,4-dimethoxybenzoic acid), o-veratric acid (2,3-dimethoxybenzoic acid), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinic acid (4,5-dimethoxyphthalic acid), gallic acid (3,4,5-trihydroxybenzoic acid), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid), asaronic acid (2,4,5-trimethoxybenzoic acid), mandelic acid (hydroxy(phenyl)acetic acid), vanillylmandelic acid (hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid ((4-methoxyphenyl)acetic acid), homogentisic acid ((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid ((3,4-dihydroxyphenyl)acetic acid), homovanillic acid ((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid ((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid ((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid ((2,3-dimethoxyphenyl)acetic acid), homophthalic acid (2-(carboxymethyl)benzoic acid), homoisophthalic acid (3-(carboxymethyl)benzoic acid), homoterephthalic acid (4-(carboxymethyl)benzoic acid), phthalonic acid (2-(carboxycarbonyl)benzoic acid), isophthalonic acid (3-(carboxycarbonyl)benzoic acid), terephthalonic acid (4-(carboxycarbonyl)benzoic acid), benzilic acid (hydroxydiphenylacetic acid), atrolactic acid (2-hydroxy-2-phenylpropanoic acid), tropic acid (3-hydroxy-2-phenylpropanoic acid), melilotic acid (3-(2-hydroxyphenyl)propanoic acid), phloretic acid (3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid (3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid (3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid (3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid (3-(4-hydroxyphenyl)acrylic acid), umbellic acid (3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid (3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid (3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid (3-(3-hydroxy-4-methoxyphenyl)acrylic acid), and sinapic acid (3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organic cation. Examples of the metal ion include sodium ion, potassium ion, lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion, aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two or more types of ions may be used in combination.

The organic cation has a carbon chain. Examples of the organic cation include organic ammonium ions. Examples of organic ammonium ions include primary ammonium ions such as stearylammonium ion, hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion; secondary ammonium ions such as dodecyl(lauryl)ammonium ion, and octadecyl(stearyl)ammonium ion; tertiary ammonium ions such as trioctylammonium ion; and quaternary ammonium ions such as dioctyldimethylammonium ion, and distearyldimethylammonium ion. Two or more types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, a magnesium salt, an aluminum salt, a zinc salt, an iron salt, a copper salt, a nickel salt, or a cobalt salt of caprylic acid (octanoic acid), lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, or behenic acid. Zinc salts of carboxylic acids are particularly preferred. Specific examples of preferable carboxylates include zinc octoate, zinc laurate, zinc myristate, and zinc stearate. A particularly preferable carboxylate is zinc octoate.

In light of linearity of the hardness distribution of the inner envelope layer 12, the amount of the acid and/or the salt (d) is preferably equal to or greater than 0.5 parts by weight, more preferably equal to or greater than 1.0 parts by weight, and particularly preferably equal to or greater than 2.0 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 40 parts by weight, more preferably equal to or less than 30 parts by weight, and particularly preferably equal to or less than 20 parts by weight, per 100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/or the salt (d) in the rubber composition is preferably equal to or greater than 3/7 but equal to or less than 9/1, and particularly preferably equal to or greater than 4/6 but equal to or less than 8/2. From the rubber composition in which this weight ratio is within the above range, the core 4 whose hardness linearly increases from its central point toward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zinc acrylate whose surface is coated with stearic acid or zinc stearate for the purpose of improving dispersibility to rubber is present. In the present invention, when the rubber composition includes this zinc acrylate, this coating material is not included in the concept of the acid and/or the salt (d).

Preferably, the rubber composition further includes an organic sulfur compound (e). The organic sulfur compound (e) can contribute to control of: the linearity of the hardness distribution of the inner envelope layer 12; and the degree of the outer-hard/inner-soft structure. An example of the organic sulfur compound (e) is an organic compound having a thiol group or a polysulfide linkage having 2 to 4 sulfur atoms. A metal salt of this organic compound is also included in the organic sulfur compound (e). Examples of the organic sulfur compound (e) include aliphatic compounds such as aliphatic thiols, aliphatic thiocarboxylic acids, aliphatic dithiocarboxylic acids, and aliphatic polysulfides; heterocyclic compounds; alicyclic compounds such as alicyclic thiols, alicyclic thiocarboxylic acids, alicyclic dithiocarboxylic acids, and alicyclic polysulfides; and aromatic compounds. Specific examples of the organic sulfur compound (e) include thiophenols, thionaphthols, polysulfides, thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, thiurams, dithiocarbamates, and thiazoles. Preferable organic sulfur compounds (e) are thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.

Specific examples of the organic sulfur compound (e) are represented by the following chemical formulas (1) to (4).

In the chemical formula (1), R1 to R5 each represent H or a substituent.

In the chemical formula (2), R1 to R10 each represent H or a substituent.

In the chemical formula (3), R1 to R5 each represent H or a substituent, and M1 represents a monovalent metal atom.

In the chemical formula (4), R1 to R10 each represent H or a substituent, and M2 represents a bivalent metal atom.

In the formulas (1) to (4), each substituent is at least one group selected from the group consisting of a halogen group (F, Cl, Br, I), an alkyl group, a carboxyl group (—COOH), an ester (—COOR) of a carboxyl group, a formyl group (—CHO), an acyl group (—COR), a carbonyl halide group (—COX), a sulfo group (—SO₃H), an ester(—SO₃R) of a sulfo group, a sulfonyl halide group (—SO₂X), a sulfino group (—SO₂H), an alkylsulfinyl group (—SOR), a carbamoyl group (—CONH₂), an alkyl halide group, a cyano group (—CN), and an alkoxy group (—OR).

Examples of the organic sulfur compound represented by the chemical formula (1) include thiophenol; thiophenols substituted with halogen groups, such as 4-fluorothiophenol, 2,5-difluorothiophenol, 2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol, pentafluorothiophenol, 2-chlorothiophenol, 4-chlorothiophenol, 2,4-dichlorothiophenol, 2,5-dichlorothiophenol, 2,6-dichlorothiophenol, 2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol, pentachlorothiophenol, 4-bromothiophenol, 2,5-dibromothiophenol, 2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol, pentabromothiophenol, 4-iodothiophenol, 2,5-diiodothiophenol, 2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, and pentaiodothiophenol; thiophenols substituted with alkyl groups, such as 4-methylthiophenol, 2,4,5-trimethylthiophenol, pentamethylthiophenol, 4-t-butylthiophenol, 2,4,5-tri-t-butylthiophenol, and penta-t-butylthiophenol; thiophenols substituted with carboxyl groups, such as 4-carboxythiophenol, 2,4,6-tricarboxythiophenol, and pentacarboxythiophenol; thiophenols substituted with alkoxycarbonyl groups, such as 4-methoxycarbonylthiophenol, 2,4,6-trimethoxycarbonylthiophenol, and pentamethoxycarbonylthiophenol; thiophenols substituted with formyl groups, such as 4-formylthiophenol, 2,4,6-triformylthiophenol, and pentaformylthiophenol; thiophenols substituted with acyl groups, such as 4-acetylthiophenol, 2,4,6-triacetylthiophenol, and pentaacetylthiophenol; thiophenols substituted with carbonyl halide groups, such as 4-chlorocarbonylthiophenol, 2,4,6-tri(chlorocarbonyl)thiophenol, and penta(chlorocarbonyl)thiophenol; thiophenols substituted with sulfo groups, such as 4-sulfothiophenol, 2,4,6-trisulfothiophenol, and pentasulfothiophenol; thiophenols substituted with alkoxysulfonyl groups, such as 4-methoxysulfonylthiophenol, 2,4,6-trimethoxysulfonylthiophenol, and pentamethoxysulfonylthiophenol; thiophenols substituted with sulfonyl halide groups, such as 4-chlorosulfonylthiophenol, 2,4,6-tri(chlorosulfonyl)thiophenol, and penta(chlorosulfonyl)thiophenol; thiophenols substituted with sulfino groups, such as 4-sulfinothiophenol, 2,4,6-trisulfinothiophenol, and pentasulfinothiophenol; thiophenols substituted with alkylsulfinyl groups, such as 4-methylsulfinylthiophenol, 2,4,6-tri(methylsulfinyl)thiophenol, and penta(methylsulfinyl)thiophenol; thiophenols substituted with carbamoyl groups, such as 4-carbamoylthiophenol, 2,4,6-tricarbamoylthiophenol, and pentacarbamoylthiophenol; thiophenols substituted with alkyl halide groups, such as 4-trichloromethylthiophenol, 2,4,6-tri(trichloromethyl)thiophenol, and penta(trichloromethyl)thiophenol; thiophenols substituted with cyano groups, such as 4-cyanothiophenol, 2,4,6-tricyanothiophenol, and pentacyanothiophenol; and thiophenols substituted with alkoxy groups, such as 4-methoxythiophenol, 2,4,6-trimethoxythiophenol, and pentamethoxythiophenol. Each of these thiophenols is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (1) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include 4-chloro-2-nitrothiophenol, 4-chloro-2-aminothiophenol, 4-chloro-2-hydroxythiophenol, 4-chloro-2-phenylthiothiophenol, 4-methyl-2-nitrothiophenol, 4-methyl-2-aminothiophenol, 4-methyl-2-hydroxythiophenol, 4-methyl-2-phenylthiothiophenol, 4-carboxy-2-nitrothiophenol, 4-carboxy-2-aminothiophenol, 4-carboxy-2-hydroxythiophenol, 4-carboxy-2-phenylthiothiophenol, 4-methoxycarbonyl-2-nitrothiophenol, 4-methoxycarbonyl-2-aminothiophenol, 4-methoxycarbonyl-2-hydroxythiophenol, 4-methoxycarbonyl-2-phenylthiothiophenol, 4-formyl-2-nitrothiophenol, 4-formyl-2-aminothiophenol, 4-formyl-2-hydroxythiophenol, 4-formyl-2-phenylthiothiophenol, 4-acetyl-2-nitrothiophenol, 4-acetyl-2-aminothiophenol, 4-acetyl-2-hydroxythiophenol, 4-acetyl-2-phenylthiothiophenol, 4-chlorocarbonyl-2-nitrothiophenol, 4-chlorocarbonyl-2-aminothiophenol, 4-chlorocarbonyl-2-hydroxythiophenol, 4-chlorocarbonyl-2-phenylthiothiophenol, 4-sulfo-2-nitrothiophenol, 4-sulfo-2-aminothiophenol, 4-sulfo-2-hydroxythiophenol, 4-sulfo-2-phenylthiothiophenol, 4-methoxysulfonyl-2-nitrothiophenol, 4-methoxysulfonyl-2-aminothiophenol, 4-methoxysulfonyl-2-hydroxythiophenol, 4-methoxysulfonyl-2-phenylthiothiophenol, 4-chlorosulfonyl-2-nitrothiophenol, 4-chlorosulfonyl-2-aminothiophenol, 4-chlorosulfonyl-2-hydroxythiophenol, 4-chlorosulfonyl-2-phenylthiothiophenol, 4-sulfino-2-nitrothiophenol, 4-sulfino-2-aminothiophenol, 4-sulfino-2-hydroxythiophenol, 4-sulfino-2-phenylthiothiophenol, 4-methylsulfinyl-2-nitrothiophenol, 4-methylsulfinyl-2-aminothiophenol, 4-methylsulfinyl-2-hydroxythiophenol, 4-methylsulfinyl-2-phenylthiothiophenol, 4-carbamoyl-2-nitrothiophenol, 4-carbamoyl-2-aminothiophenol, 4-carbamoyl-2-hydroxythiophenol, 4-carbamoyl-2-phenylthiothiophenol, 4-trichloromethyl-2-nitrothiophenol, 4-trichloromethyl-2-aminothiophenol, 4-trichloromethyl-2-hydroxythiophenol, 4-trichloromethyl-2-phenylthiothiophenol, 4-cyano-2-nitrothiophenol, 4-cyano-2-aminothiophenol, 4-cyano-2-hydroxythiophenol, 4-cyano-2-phenylthiothiophenol, 4-methoxy-2-nitrothiophenol, 4-methoxy-2-aminothiophenol, 4-methoxy-2-hydroxythiophenol, and 4-methoxy-2-phenylthiothiophenol.

Still another example of the organic sulfur compound represented by the chemical formula (1) is a compound substituted with two or more types of substituents. Specific examples of the compound include 4-acetyl-2-chlorothiophenol, 4-acetyl-2-methylthiophenol, 4-acetyl-2-carboxythiophenol, 4-acetyl-2-methoxycarbonylthiophenol, 4-acetyl-2-formylthiophenol, 4-acetyl-2-chlorocarbonylthiophenol, 4-acetyl-2-sulfothiophenol, 4-acetyl-2-methoxysulfonylthiophenol, 4-acetyl-2-chlorosulfonylthiophenol, 4-acetyl-2-sulfinothiophenol, 4-acetyl-2-methylsulfinylthiophenol, 4-acetyl-2-carbamoylthiophenol, 4-acetyl-2-trichloromethylthiophenol, 4-acetyl-2-cyanothiophenol, and 4-acetyl-2-methoxythiophenol.

Examples of the organic sulfur compound represented by the chemical formula (2) include diphenyl disulfide; diphenyl disulfides substituted with halogen groups, such as bis(4-fluorophenyl)disulfide, bis(2,5-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide, bis(2,4,5,6-tetrafluorophenyl)disulfide, bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide, bis(2,5-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide, bis(2,4,5,6-tetrachlorophenyl)disulfide, bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide, bis(2,5-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide, bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide, bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide, bis(2,4,5-triiodophenyl)disulfide, bis(2,4,5,6-tetraiodophenyl)disulfide, and bis(pentaiodophenyl)disulfide; diphenyl disulfides substituted with alkyl groups, such as bis(4-methylphenyl)disulfide, bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide, bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide, and bis(penta-t-butylphenyl)disulfide; diphenyl disulfides substituted with carboxyl groups, such as bis(4-carboxyphenyl)disulfide, bis(2,4,6-tricarboxyphenyl)disulfide, and bis(pentacarboxyphenyl)disulfide; diphenyl disulfides substituted with alkoxycarbonyl groups, such as bis(4-methoxycarbonylphenyl)disulfide, bis(2,4,6-trimethoxycarbonylphenyl)disulfide, and bis(pentamethoxycarbonylphenyl)disulfide; diphenyl disulfides substituted with formyl groups, such as bis(4-formylphenyl)disulfide, bis(2,4,6-triformylphenyl)disulfide, and bis(pentaformylphenyl)disulfide; diphenyl disulfides substituted with acyl groups, such as bis(4-acetylphenyl)disulfide, bis(2,4,6-triacetylphenyl)disulfide, and bis(pentaacetylphenyl)disulfide; diphenyl disulfides substituted with carbonyl halide groups, such as bis(4-chlorocarbonylphenyl)disulfide, bis(2,4,6-tri(chlorocarbonyl)phenyl)disulfide, and bis(penta(chlorocarbonyl)phenyl)disulfide; diphenyl disulfides substituted with sulfo groups, such as bis(4-sulfophenyl)disulfide, bis(2,4,6-trisulfophenyl)disulfide, and bis(pentasulfophenyl)disulfide; diphenyl disulfides substituted with alkoxysulfonyl groups, such as bis(4-methoxysulfonylphenyl)disulfide, bis(2,4,6-trimethoxysulfonylphenyl)disulfide, and bis(pentamethoxysulfonylphenyl)disulfide; diphenyl disulfides substituted with sulfonyl halide groups, such as bis(4-chlorosulfonylphenyl)disulfide, bis(2,4,6-tri(chlorosulfonyl)phenyl)disulfide, and bis(penta(chlorosulfonyl)phenyl)disulfide; diphenyl disulfides substituted with sulfino groups, such as bis(4-sulfinophenyl)disulfide, bis(2,4,6-trisulfinophenyl)disulfide, and bis(pentasulfinophenyl)disulfide; diphenyl disulfides substituted with alkylsulfinyl groups, such as bis(4-methylsulfinylphenyl)disulfide, bis(2,4,6-tri(methylsulfinyl)phenyl)disulfide, and bis(penta(methylsulfinyl)phenyl)disulfide; diphenyl disulfides substituted with carbamoyl groups, such as bis(4-carbamoylphenyl)disulfide, bis(2,4,6-tricarbamoylphenyl)disulfide, and bis(pentacarbamoylphenyl)disulfide; diphenyl disulfides substituted with alkyl halide groups, such as bis(4-trichloromethylphenyl)disulfide, bis(2,4,6-tri(trichloromethyl)phenyl)disulfide, and bis(penta(trichloromethyl)phenyl)disulfide; diphenyl disulfides substituted with cyano groups, such as bis(4-cyanophenyl)disulfide, bis(2,4,6-tricyanophenyl)disulfide, and bis(pentacyanophenyl)disulfide; and diphenyl disulfides substituted with alkoxy groups, such as bis(4-methoxyphenyl)disulfide, bis(2,4,6-trimethoxyphenyl)disulfide, and bis(pentamethoxyphenyl)disulfide. Each of these diphenyl disulfides is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (2) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include bis(4-chloro-2-nitrophenyl)disulfide, bis(4-chloro-2-aminophenyl)disulfide, bis(4-chloro-2-hydroxyphenyl)disulfide, bis(4-chloro-2-phenylthiophenyl)disulfide, bis(4-methyl-2-nitrophenyl)disulfide, bis(4-methyl-2-aminophenyl)disulfide, bis(4-methyl-2-hydroxyphenyl)disulfide, bis(4-methyl-2-phenylthiophenyl)disulfide, bis(4-carboxy-2-nitrophenyl)disulfide, bis(4-carboxy-2-aminophenyl)disulfide, bis(4-carboxy-2-hydroxyphenyl)disulfide, bis(4-carboxy-2-phenylthiophenyl)disulfide, bis(4-methoxycarbonyl-2-nitrophenyl)disulfide, bis(4-methoxycarbonyl-2-aminophenyl)disulfide, bis(4-methoxycarbonyl-2-hydroxyphenyl)disulfide, bis(4-methoxycarbonyl-2-phenylthiophenyl)disulfide, bis(4-formyl-2-nitrophenyl)disulfide, bis(4-formyl-2-aminophenyl)disulfide, bis(4-formyl-2-hydroxyphenyl)disulfide, bis(4-formyl-2-phenylthiophenyl)disulfide, bis(4-acetyl-2-nitrophenyl)disulfide, bis(4-acetyl-2-aminophenyl)disulfide, bis(4-acetyl-2-hydroxyphenyl)disulfide, bis(4-acetyl-2-phenylthiophenyl)disulfide, bis(4-chlorocarbonyl-2-nitrophenyl)disulfide, bis(4-chlorocarbonyl-2-aminophenyl)disulfide, bis(4-chlorocarbonyl-2-hydroxyphenyl)disulfide, bis(4-chlorocarbonyl-2-phenylthiophenyl)disulfide, bis(4-sulfo-2-nitrophenyl)disulfide, bis(4-sulfo-2-aminophenyl)disulfide, bis(4-sulfo-2-hydroxyphenyl)disulfide, bis(4-sulfo-2-phenylthiophenyl)disulfide, bis(4-methoxysulfonyl-2-nitrophenyl)disulfide, bis(4-methoxysulfonyl-2-aminophenyl)disulfide, bis(4-methoxysulfonyl-2-hydroxyphenyl)disulfide, bis(4-methoxysulfonyl-2-phenylthiophenyl)disulfide, bis(4-chlorosulfonyl-2-nitrophenyl)disulfide, bis(4-chlorosulfonyl-2-aminophenyl)disulfide, bis(4-chlorosulfonyl-2-hydroxyphenyl)disulfide, bis(4-chlorosulfonyl-2-phenylthiophenyl)disulfide, bis(4-sulfino-2-nitrophenyl)disulfide, bis(4-sulfino-2-aminophenyl)disulfide, bis(4-sulfino-2-hydroxyphenyl)disulfide, bis(4-sulfino-2-phenylthiophenyl)disulfide, bis(4-methylsulfinyl-2-nitrophenyl)disulfide, bis(4-methylsulfinyl-2-aminophenyl)disulfide, bis(4-methylsulfinyl-2-hydroxyphenyl)disulfide, bis(4-methylsulfinyl-2-phenylthiophenyl)disulfide, bis(4-carbamoyl-2-nitrophenyl)disulfide, bis(4-carbamoyl-2-aminophenyl)disulfide, bis(4-carbamoyl-2-hydroxyphenyl)disulfide, bis(4-carbamoyl-2-phenylthiophenyl)disulfide, bis(4-trichloromethyl-2-nitrophenyl)disulfide, bis(4-trichloromethyl-2-aminophenyl)disulfide, bis(4-trichloromethyl-2-hydroxyphenyl)disulfide, bis(4-trichloromethyl-2-phenylthiophenyl)disulfide, bis(4-cyano-2-nitrophenyl)disulfide, bis(4-cyano-2-aminophenyl)disulfide, bis(4-cyano-2-hydroxyphenyl)disulfide, bis(4-cyano-2-phenylthiophenyl)disulfide, bis(4-methoxy-2-nitrophenyl)disulfide, bis(4-methoxy-2-aminophenyl)disulfide, bis(4-methoxy-2-hydroxyphenyl)disulfide, and bis(4-methoxy-2-phenylthiophenyl)disulfide.

Still another example of the organic sulfur compound represented by the chemical formula (2) is a compound substituted with two or more types of substituents. Specific examples of the compound include bis(4-acetyl-2-chlorophenyl)disulfide, bis(4-acetyl-2-methylphenyl)disulfide, bis(4-acetyl-2-carboxyphenyl)disulfide, bis(4-acetyl-2-methoxycarbonylphenyl)disulfide, bis(4-acetyl-2-formylphenyl)disulfide, bis(4-acetyl-2-chlorocarbonylphenyl)disulfide, bis(4-acetyl-2-sulfophenyl)disulfide, bis(4-acetyl-2-methoxysulfonylphenyl)disulfide, bis(4-acetyl-2-chlorosulfonylphenyl)disulfide, bis(4-acetyl-2-sulfinophenyl)disulfide, bis(4-acetyl-2-methylsulfinylphenyl)disulfide, bis(4-acetyl-2-carbamoylphenyl)disulfide, bis(4-acetyl-2-trichloromethylphenyl)disulfide, bis(4-acetyl-2-cyanophenyl)disulfide, and bis(4-acetyl-2-methoxyphenyl)disulfide.

Examples of the organic sulfur compound represented by the chemical formula (3) include thiophenol sodium salt; thiophenol sodium salts substituted with halogen groups, such as 4-fluorothiophenol sodium salt, 2,5-difluorothiophenol sodium salt, 2,4,5-trifluorothiophenol sodium salt, 2,4,5,6-tetrafluorothiophenol sodium salt, pentafluorothiophenol sodium salt, 4-chlorothiophenol sodium salt, 2,5-dichlorothiophenol sodium salt, 2,4,5-trichlorothiophenol sodium salt, 2,4,5,6-tetrachlorothiophenol sodium salt, pentachlorothiophenol sodium salt, 4-bromothiophenol sodium salt, 2,5-dibromothiophenol sodium salt, 2,4,5-tribromothiophenol sodium salt, 2,4,5,6-tetrabromothiophenol sodium salt, pentabromothiophenol sodium salt, 4-iodothiophenol sodium salt, 2,5-diiodothiophenol sodium salt, 2,4,5-triiodothiophenol sodium salt, 2,4,5,6-tetraiodothiophenol sodium salt, and pentaiodothiophenol sodium salt; thiophenol sodium salts substituted with alkyl groups, such as 4-methylthiophenol sodium salt, 2,4,5-trimethylthiophenol sodium salt, pentamethylthiophenol sodium salt, 4-t-butylthiophenol sodium salt, 2,4,5-tri-t-butylthiophenol sodium salt, and penta(t-butyl)thiophenol sodium salt; thiophenol sodium salts substituted with carboxyl groups, such as 4-carboxythiophenol sodium salt, 2,4,6-tricarboxythiophenol sodium salt, and pentacarboxythiophenol sodium salt; thiophenol sodium salts substituted with alkoxycarbonyl groups, such as 4-methoxycarbonylthiophenol sodium salt, 2,4,6-trimethoxycarbonylthiophenol sodium salt, and pentamethoxycarbonylthiophenol sodium salt; thiophenol sodium salts substituted with formyl groups, such as 4-formylthiophenol sodium salt, 2,4,6-triformylthiophenol sodium salt, and pentaformylthiophenol sodium salt; thiophenol sodium salts substituted with acyl groups, such as 4-acetylthiophenol sodium salt, 2,4,6-triacetylthiophenol sodium salt, and pentaacetylthiophenol sodium salt; thiophenol sodium salts substituted with carbonyl halide groups, such as 4-chlorocarbonylthiophenol sodium salt, 2,4,6-tri(chlorocarbonyl)thiophenol sodium salt, and penta(chlorocarbonyl)thiophenol sodium salt; thiophenol sodium salts substituted with sulfo groups, such as 4-sulfothiophenol sodium salt, 2,4,6-trisulfothiophenol sodium salt, and pentasulfothiophenol sodium salt; thiophenol sodium salts substituted with alkoxysulfonyl groups, such as 4-methoxysulfonylthiophenol sodium salt, 2,4,6-trimethoxysulfonylthiophenol sodium salt, and pentamethoxysulfonylthiophenol sodium salt; thiophenol sodium salts substituted with sulfonyl halide groups, such as 4-chlorosulfonylthiophenol sodium salt, 2,4,6-tri(chlorosulfonyl)thiophenol sodium salt, and penta(chlorosulfonyl)thiophenol sodium salt; thiophenol sodium salts substituted with sulfino groups, such as 4-sulfinothiophenol sodium salt, 2,4,6-trisulfinothiophenol sodium salt, and pentasulfinothiophenol sodium salt; thiophenol sodium salts substituted with alkylsulfinyl groups, such as 4-methylsulfinylthiophenol sodium salt, 2,4,6-tri(methylsulfinyl)thiophenol sodium salt, and penta(methylsulfinyl)thiophenol sodium salt; thiophenol sodium salts substituted with carbamoyl groups, such as 4-carbamoylthiophenol sodium salt, 2,4,6-tricarbamoylthiophenol sodium salt, and pentacarbamoylthiophenol sodium salt; thiophenol sodium salts substituted with alkyl halide groups, such as 4-trichloromethylthiophenol sodium salt, 2,4,6-tri(trichloromethyl)thiophenol sodium salt, and penta(trichloromethyl)thiophenol sodium salt; thiophenol sodium salts substituted with cyano groups, such as 4-cyanothiophenol sodium salt, 2,4,6-tricyanothiophenol sodium salt, and pentacyanothiophenol sodium salt; and thiophenol sodium salts substituted with alkoxy groups, such as 4-methoxythiophenol sodium salt, 2,4,6-trimethoxythiophenol sodium salt, and pentamethoxythiophenol sodium salt. Each of these thiophenol sodium salts is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (3) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include 4-chloro-2-nitrothiophenol sodium salt, 4-chloro-2-aminothiophenol sodium salt, 4-chloro-2-hydroxythiophenol sodium salt, 4-chloro-2-phenylthiothiophenol sodium salt, 4-methyl-2-nitrothiophenol sodium salt, 4-methyl-2-aminothiophenol sodium salt, 4-methyl-2-hydroxythiophenol sodium salt, 4-methyl-2-phenylthiothiophenol sodium salt, 4-carboxy-2-nitrothiophenol sodium salt, 4-carboxy-2-aminothiophenol sodium salt, 4-carboxy-2-hydroxythiophenol sodium salt, 4-carboxy-2-phenylthiothiophenol sodium salt, 4-methoxycarbonyl-2-nitrothiophenol sodium salt, 4-methoxycarbonyl-2-aminothiophenol sodium salt, 4-methoxycarbonyl-2-hydroxythiophenol sodium salt, 4-methoxycarbonyl-2-phenylthiothiophenol sodium salt, 4-formyl-2-nitrothiophenol sodium salt, 4-formyl-2-aminothiophenol sodium salt, 4-formyl-2-hydroxythiophenol sodium salt, 4-formyl-2-phenylthiothiophenol sodium salt, 4-acetyl-2-nitrothiophenol sodium salt, 4-acetyl-2-aminothiophenol sodium salt, 4-acetyl-2-hydroxythiophenol sodium salt, 4-acetyl-2-phenylthiothiophenol sodium salt, 4-chlorocarbonyl-2-nitrothiophenol sodium salt, 4-chlorocarbonyl-2-aminothiophenol sodium salt, 4-chlorocarbonyl-2-hydroxythiophenol sodium salt, 4-chlorocarbonyl-2-phenylthiothiophenol sodium salt, 4-sulfo-2-nitrothiophenol sodium salt, 4-sulfo-2-aminothiophenol sodium salt, 4-sulfo-2-hydroxythiophenol sodium salt, 4-sulfo-2-phenylthiothiophenol sodium salt, 4-methoxysulfonyl-2-nitrothiophenol sodium salt, 4-methoxysulfonyl-2-aminothiophenol sodium salt, 4-methoxysulfonyl-2-hydroxythiophenol sodium salt, 4-methoxysulfonyl-2-phenylthiothiophenol sodium salt, 4-chlorosulfonyl-2-nitrothiophenol sodium salt, 4-chlorosulfonyl-2-aminothiophenol sodium salt, 4-chlorosulfonyl-2-hydroxythiophenol sodium salt, 4-chlorosulfonyl-2-phenylthiothiophenol sodium salt, 4-sulfino-2-nitrothiophenol sodium salt, 4-sulfino-2-aminothiophenol sodium salt, 4-sulfino-2-hydroxythiophenol sodium salt, 4-sulfino-2-phenylthiothiophenol sodium salt, 4-methylsulfinyl-2-nitrothiophenol sodium salt, 4-methylsulfinyl-2-aminothiophenol sodium salt, 4-methylsulfinyl-2-hydroxythiophenol sodium salt, 4-methylsulfinyl-2-phenylthiothiophenol sodium salt, 4-carbamoyl-2-nitrothiophenol sodium salt, 4-carbamoyl-2-aminothiophenol sodium salt, 4-carbamoyl-2-hydroxythiophenol sodium salt, 4-carbamoyl-2-phenylthiothiophenol sodium salt, 4-trichloromethyl-2-nitrothiophenol sodium salt, 4-trichloromethyl-2-aminothiophenol sodium salt, 4-trichloromethyl-2-hydroxythiophenol sodium salt, 4-trichloromethyl-2-phenylthiothiophenol sodium salt, 4-cyano-2-nitrothiophenol sodium salt, 4-cyano-2-aminothiophenol sodium salt, 4-cyano-2-hydroxythiophenol sodium salt, 4-cyano-2-phenylthiothiophenol sodium salt, 4-methoxy-2-nitrothiophenol sodium salt, 4-methoxy-2-aminothiophenol sodium salt, 4-methoxy-2-hydroxythiophenol sodium salt, and 4-methoxy-2-phenylthiothiophenol sodium salt.

Still another example of the organic sulfur compound represented by the chemical formula (3) is a compound substituted with two or more types of substituents. Specific examples of the compound include 4-acetyl-2-chlorothiophenol sodium salt, 4-acetyl-2-methylthiophenol sodium salt, 4-acetyl-2-carboxythiophenol sodium salt, 4-acetyl-2-methoxycarbonylthiophenol sodium salt, 4-acetyl-2-formylthiophenol sodium salt, 4-acetyl-2-chlorocarbonylthiophenol sodium salt, 4-acetyl-2-sulfothiophenol sodium salt, 4-acetyl-2-methoxysulfonylthiophenol sodium salt, 4-acetyl-2-chlorosulfonylthiophenol sodium salt, 4-acetyl-2-sulfinothiophenol sodium salt, 4-acetyl-2-methylsulfinylthiophenol sodium salt, 4-acetyl-2-carbamoylthiophenol sodium salt, 4-acetyl-2-trichloromethylthiophenol sodium salt, 4-acetyl-2-cyanothiophenol sodium salt, and 4-acetyl-2-methoxythiophenol sodium salt. Examples of the monovalent metal represented by M1 in the chemical formula (3) include sodium, lithium, potassium, copper (I), and silver (I).

Examples of the organic sulfur compound represented by the chemical formula (4) include thiophenol zinc salt; thiophenol zinc salts substituted with halogen groups, such as 4-fluorothiophenol zinc salt, 2,5-difluorothiophenol zinc salt, 2,4,5-trifluorothiophenol zinc salt, 2,4,5,6-tetrafluorothiophenol zinc salt, pentafluorothiophenol zinc salt, 4-chlorothiophenol zinc salt, 2,5-dichlorothiophenol zinc salt, 2,4,5-trichlorothiophenol zinc salt, 2,4,5,6-tetrachlorothiophenol zinc salt, pentachlorothiophenol zinc salt, 4-bromothiophenol zinc salt, 2,5-dibromothiophenol zinc salt, 2,4,5-tribromothiophenol zinc salt, 2,4,5,6-tetrabromothiophenol zinc salt, pentabromothiophenol zinc salt, 4-iodothiophenol zinc salt, 2,5-diiodothiophenol zinc salt, 2,4,5-triiodothiophenol zinc salt, 2,4,5,6-tetraiodothiophenol zinc salt, and pentaiodothiophenol zinc salt; thiophenol zinc salts substituted with alkyl groups, such as 4-methylthiophenol zinc salt, 2,4,5-trimethylthiophenol zinc salt, pentamethylthiophenol zinc salt, 4-t-butylthiophenol zinc salt, 2,4,5-tri-t-butylthiophenol zinc salt, and penta-t-butylthiophenol zinc salt; thiophenol zinc salts substituted with carboxyl groups, such as 4-carboxythiophenol zinc salt, 2,4,6-tricarboxythiophenol zinc salt, and pentacarboxythiophenol zinc salt; thiophenol zinc salts substituted with alkoxycarbonyl groups, such as 4-methoxycarbonylthiophenol zinc salt, 2,4,6-trimethoxycarbonylthiophenol zinc salt, and pentamethoxycarbonylthiophenol zinc salt; thiophenol zinc salts substituted with formyl groups, such as 4-formylthiophenol zinc salt, 2,4,6-triformylthiophenol zinc salt, and pentaformylthiophenol zinc salt; thiophenol zinc salts substituted with acyl groups, such as 4-acetylthiophenol zinc salt, 2,4,6-triacetylthiophenol zinc salt, and pentaacetylthiophenol zinc salt; thiophenol zinc salts substituted with carbonyl halide groups, such as 4-chlorocarbonylthiophenol zinc salt, 2,4,6-tri(chlorocarbonyl)thiophenol zinc salt, and penta(chlorocarbonyl)thiophenol zinc salt; thiophenol zinc salts substituted with sulfo groups, such as 4-sulfothiophenol zinc salt, 2,4,6-trisulfothiophenol zinc salt, and pentasulfothiophenol zinc salt; thiophenol zinc salts substituted with alkoxysulfonyl groups, such as 4-methoxysulfonylthiophenol zinc salt, 2,4,6-trimethoxysulfonylthiophenol zinc salt, and pentamethoxysulfonylthiophenol zinc salt; thiophenol zinc salts substituted with sulfonyl halide groups, such as 4-chlorosulfonylthiophenol zinc salt, 2,4,6-tri(chlorosulfonyl)thiophenol zinc salt, and penta(chlorosulfonyl)thiophenol zinc salt; thiophenol zinc salts substituted with sulfino groups, such as 4-sulfinothiophenol zinc salt, 2,4,6-trisulfinothiophenol zinc salt, and pentasulfinothiophenol zinc salt; thiophenol zinc salts substituted with alkylsulfinyl groups, such as 4-methylsulfinylthiophenol zinc salt, 2,4,6-tri(methylsulfinyl)thiophenol zinc salt, and penta(methylsulfinyl)thiophenol zinc salt; thiophenol zinc salts substituted with carbamoyl groups, such as 4-carbamoylthiophenol zinc salt, 2,4,6-tricarbamoylthiophenol zinc salt, and pentacarbamoylthiophenol zinc salt; thiophenol zinc salts substituted with alkyl halide groups, such as 4-trichloromethylthiophenol zinc salt, 2,4,6-tri(trichloromethyl)thiophenol zinc salt, and penta(trichloromethyl)thiophenol zinc salt; thiophenol zinc salts substituted with cyano groups, such as 4-cyanothiophenol zinc salt, 2,4,6-tricyanothiophenol zinc salt, and pentacyanothiophenol zinc salt; and thiophenol zinc salts substituted with alkoxy groups, such as 4-methoxythiophenol zinc salt, 2,4,6-trimethoxythiophenol zinc salt, and pentamethoxythiophenol zinc salt. Each of these thiophenol zinc salts is substituted with one type of substituent.

Another example of the organic sulfur compound represented by the chemical formula (4) is a compound substituted with at least one type of the above substituents and another substituent. Examples of the other substituent include a nitro group (—NO₂), an amino group (—NH₂), a hydroxyl group (—OH), and a phenylthio group (—SPh). Specific examples of the compound include 4-chloro-2-nitrothiophenol zinc salt, 4-chloro-2-aminothiophenol zinc salt, 4-chloro-2-hydroxythiophenol zinc salt, 4-chloro-2-phenylthiothiophenol zinc salt, 4-methyl-2-nitrothiophenol zinc salt, 4-methyl-2-aminothiophenol zinc salt, 4-methyl-2-hydroxythiophenol zinc salt, 4-methyl-2-phenylthiothiophenol zinc salt, 4-carboxy-2-nitrothiophenol zinc salt, 4-carboxy-2-aminothiophenol zinc salt, 4-carboxy-2-hydroxythiophenol zinc salt, 4-carboxy-2-phenylthiothiophenol zinc salt, 4-methoxycarbonyl-2-nitrothiophenol zinc salt, 4-methoxycarbonyl-2-aminothiophenol zinc salt, 4-methoxycarbonyl-2-hydroxythiophenol zinc salt, 4-methoxycarbonyl-2-phenylthiothiophenol zinc salt, 4-formyl-2-nitrothiophenol zinc salt, 4-formyl-2-aminothiophenol zinc salt, 4-formyl-2-hydroxythiophenol zinc salt, 4-formyl-2-phenylthiothiophenol zinc salt, 4-acetyl-2-nitrothiophenol zinc salt, 4-acetyl-2-aminothiophenol zinc salt, 4-acetyl-2-hydroxythiophenol zinc salt, 4-acetyl-2-phenylthiothiophenol zinc salt, 4-chlorocarbonyl-2-nitrothiophenol zinc salt, 4-chlorocarbonyl-2-aminothiophenol zinc salt, 4-chlorocarbonyl-2-hydroxythiophenol zinc salt, 4-chlorocarbonyl-2-phenylthiothiophenol zinc salt, 4-sulfo-2-nitrothiophenol zinc salt, 4-sulfo-2-aminothiophenol zinc salt, 4-sulfo-2-hydroxythiophenol zinc salt, 4-sulfo-2-phenylthiothiophenol zinc salt, 4-methoxysulfonyl-2-nitrothiophenol zinc salt, 4-methoxysulfonyl-2-aminothiophenol zinc salt, 4-methoxysulfonyl-2-hydroxythiophenol zinc salt, 4-methoxysulfonyl-2-phenylthiothiophenol zinc salt, 4-chlorosulfonyl-2-nitrothiophenol zinc salt, 4-chlorosulfonyl-2-aminothiophenol zinc salt, 4-chlorosulfonyl-2-hydroxythiophenol zinc salt, 4-chlorosulfonyl-2-phenylthiothiophenol zinc salt, 4-sulfino-2-nitrothiophenol zinc salt, 4-sulfino-2-aminothiophenol zinc salt, 4-sulfino-2-hydroxythiophenol zinc salt, 4-sulfino-2-phenylthiothiophenol zinc salt, 4-methylsulfinyl-2-nitrothiophenol zinc salt, 4-methylsulfinyl-2-aminothiophenol zinc salt, 4-methylsulfinyl-2-hydroxythiophenol zinc salt, 4-methylsulfinyl-2-phenylthiothiophenol zinc salt, 4-carbamoyl-2-nitrothiophenol zinc salt, 4-carbamoyl-2-aminothiophenol zinc salt, 4-carbamoyl-2-hydroxythiophenol zinc salt, 4-carbamoyl-2-phenylthiothiophenol zinc salt, 4-trichloromethyl-2-nitrothiophenol zinc salt, 4-trichloromethyl-2-aminothiophenol zinc salt, 4-trichloromethyl-2-hydroxythiophenol zinc salt, 4-trichloromethyl-2-phenylthiothiophenol zinc salt, 4-cyano-2-nitrothiophenol zinc salt, 4-cyano-2-aminothiophenol zinc salt, 4-cyano-2-hydroxythiophenol zinc salt, 4-cyano-2-phenylthiothiophenol zinc salt, 4-methoxy-2-nitrothiophenol zinc salt, 4-methoxy-2-aminothiophenol zinc salt, 4-methoxy-2-hydroxythiophenol zinc salt, and 4-methoxy-2-phenylthiothiophenol zinc salt.

Still another example of the organic sulfur compound represented by the chemical formula (4) is a compound substituted with two or more types of substituents. Specific examples of the compound include 4-acetyl-2-chlorothiophenol zinc salt, 4-acetyl-2-methylthiophenol zinc salt, 4-acetyl-2-carboxythiophenol zinc salt, 4-acetyl-2-methoxycarbonylthiophenol zinc salt, 4-acetyl-2-formylthiophenol zinc salt, 4-acetyl-2-chlorocarbonylthiophenol zinc salt, 4-acetyl-2-sulfothiophenol zinc salt, 4-acetyl-2-methoxysulfonylthiophenol zinc salt, 4-acetyl-2-chlorosulfonylthiophenol zinc salt, 4-acetyl-2-sulfinothiophenol zinc salt, 4-acetyl-2-methylsulfinylthiophenol zinc salt, 4-acetyl-2-carbamoylthiophenol zinc salt, 4-acetyl-2-trichloromethylthiophenol zinc salt, 4-acetyl-2-cyanothiophenol zinc salt, and 4-acetyl-2-methoxythiophenol zinc salt. Examples of the bivalent metal represented by M2 in the chemical formula (4) include zinc, magnesium, calcium, strontium, barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel (II), zirconium (II), and tin (II).

Examples of thionaphthols include 2-thionaphthol, 1-thionaphthol, 2-chloro-1-thionaphthol, 2-bromo-1-thionaphthol, 2-fluoro-1-thionaphthol, 2-cyano-1-thionaphthol, 2-acetyl-1-thionaphthol, 1-chloro-2-thionaphthol, 1-bromo-2-thionaphthol, 1-fluoro-2-thionaphthol, 1-cyano-2-thionaphthol, 1-acetyl-2-thionaphthol, and metal salts thereof. 1-thionaphthol, 2-thionaphthol, and zinc salts thereof are preferred.

Examples of sulfenamide type organic sulfur compounds include N-cyclohexyl-2-benzothiazole sulfenamide, N-oxydiethylene-2-benzothiazole sulfenamide, and N-t-butyl-2-benzothiazole sulfenamide. Examples of thiuram type organic sulfur compounds include tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide. Examples of dithiocarbamates include zinc dimethyldithiocarbamate, zinc diethyldithiocarbamate, zinc dibutyldithiocarbamate, zinc ethylphenyldithiocarbamate, sodium dimethyldithiocarbamate, sodium diethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III) dimethyldithiocarbamate, selenium diethyldithiocarbamate, and tellurium diethyldithiocarbamate. Examples of thiazole type organic sulfur compounds include 2-mercaptobenzothiazole (MBT); dibenzothiazyl disulfide (MBTS); a sodium salt, a zinc salt, a copper salt, or a cyclohexylamine salt of 2-mercaptobenzothiazole; 2-(2,4-dinitrophenyl)mercaptobenzothiazole; and 2-(2,6-diethyl-4-morpholinothio)benzothiazole

Particularly preferable organic sulfur compounds (e) from the standpoint that an outer-hard/inner-soft structure is easily obtained are 2-thionaphthol, bis(pentabromophenyl)disulfide, and 2,6-dichlorothiophenol.

From the standpoint that an outer-hard/inner-soft structure is easily obtained, the amount of the organic sulfur compound (e) is preferably equal to or greater than 0.05 parts by weight, more preferably equal to or greater than 0.1 parts by weight, and particularly preferably equal to or greater than 0.2 parts by weight, per 100 parts by weight of the base rubber. In light of resilience performance, the amount is preferably equal to or less than 5.0 parts by weight, more preferably equal to or less than 3.0 parts by weight, and particularly preferably equal to or less than 1.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler may be included in the inner envelope layer 12. Examples of suitable fillers include zinc oxide, barium sulfate, calcium carbonate, and magnesium carbonate. The amount of the filler is determined as appropriate so that the intended specific gravity of the core 4 is accomplished. A particularly preferable filler is zinc oxide. Zinc oxide serves not only as a specific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer, a dispersant, sulfur, a vulcanization accelerator, and the like are added to the rubber composition of the inner envelope layer 12. Crosslinked rubber powder or synthetic resin powder may also be dispersed in the rubber composition.

During heating and forming of the core 4, the base rubber (a) is crosslinked by the co-crosslinking agent (b). The heat of the crosslinking reaction remains near the central point of the core 4. Thus, during heating and forming of the core 4, the temperature at the central portion is high. The temperature gradually decreases from the central point toward the surface. It is inferred that in the rubber composition, the acid reacts with the metal salt of the co-crosslinking agent (b) to bond to cation. It is inferred that in the rubber composition, the salt reacts with the metal salt of the co-crosslinking agent (b) to exchange cation. By the bonding and the exchange, metal crosslinks are broken. This bonding and exchange are likely to occur near the innermost portion of the inner envelope layer 12 where the temperature is high, and is unlikely to occur near the surface of the inner envelope layer 12. In other words, the breaking of the metal crosslinks is likely to occur near the innermost portion of the inner envelope layer 12, and is unlikely to occur near the surface of the inner envelope layer 12. As a result, the crosslinking density of the inner envelope layer 12 increases from its inside toward its outside. In the inner envelope layer 12, the hardness linearly increases from its inside toward its outside. Further, since the rubber composition includes the organic sulfur compound (e) together with the acid and/or the salt (d), the gradient of the hardness distribution can be controlled, and the degree of the outer-hard/inner-soft structure of the core 4 can be increased. The core 4 can suppress spin.

The rubber composition of the inner envelope layer 12 may not include the acid and/or the salt (d).

The outer envelope layer 14 is formed by crosslinking a rubber composition. The outer envelope layer 14 can be formed from the rubber composition described above for the inner envelope layer 12. The rubber composition can include the acid and/or the salt (d). A hardness distribution of the outer envelope layer 14 is appropriate. The rubber composition of the outer envelope layer 14 may not include the acid and/or the salt (d).

Preferably, the hardness H(0.0) at the central point of the core 4 is equal to or greater than 40.0 but equal to or less than 70.0. The golf ball 2 having a hardness H(0.0) of 40.0 or greater has excellent resilience performance. In this respect, the hardness H(0.0) is more preferably equal to or greater than 45.0 and particularly preferably equal to or greater than 47.0. In the core 4 having a hardness H(0.0) of 70.0 or less, an outer-hard/inner-soft structure can be achieved. In the golf ball 2 that includes the core 4, spin can be suppressed. In this respect, the hardness H(0.0) is more preferably equal to or less than 68.0 and particularly preferably equal to or less than 65.0.

Preferably, the hardness Hs at the surface of the core 4 is equal to or greater than 75.0 but equal to or less than 95.0. In the core 4 having a hardness Hs of 75.0 or greater, an outer-hard/inner-soft structure can be achieved. In the golf ball 2 that includes the core 4, spin can be suppressed. In this respect, the hardness Hs is more preferably equal to or greater than 80.0 and particularly preferably equal to or greater than 82.0. The golf ball 2 having a hardness Hs of 95.0 or less has excellent durability. In this respect, the hardness Hs is more preferably equal to or less than 94.0 and particularly preferably equal to or less than 92.0.

The core 4 preferably has a diameter of 38.0 mm or greater but 41.5 mm or less. The core 4 having a diameter of 38.0 mm or greater can achieve excellent resilience performance of the golf ball 2. In this respect, the diameter is more preferably equal to or greater than 38.5 mm and particularly preferably equal to or greater than 39.0 mm. In the golf ball 2 that includes the core 4 having a diameter of 41.5 mm or less, the inner cover 6 and the outer cover 8 can have sufficient thicknesses. The golf ball 2 that includes the inner cover 6 and the outer cover 8 which have large thicknesses has excellent durability. In this respect, the diameter is more preferably equal to or less than 41.0 mm and particularly preferably equal to or less than 40.5 mm.

In light of feel at impact, the core 4 has an amount of compressive deformation (Comp'n) of preferably 3.5 mm or greater and particularly preferably 3.8 mm or greater. In light of resilience performance of the core 4, the amount of compressive deformation is preferably equal to or less than 4.5 mm and particularly preferably equal to or less than 4.0 mm.

For the inner cover 6, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, polystyrenes, polyesters, polyamides, and polyolefins.

Particularly preferable base polymers are ionomer resins. The golf ball 2 that includes the inner cover 6 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the inner cover 6. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight.

Examples of preferable ionomer resins include binary copolymers formed with an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms. A preferable binary copolymer includes 80% by weight or greater but 90% by weight or less of an α-olefin, and 10% by weight or greater but 20% by weight or less of an α,β-unsaturated carboxylic acid. The binary copolymer has excellent resilience performance. Examples of other preferable ionomer resins include ternary copolymers formed with: an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbon atoms. A preferable ternary copolymer includes 70% by weight or greater but 85% by weight or less of an α-olefin, 5% by weight or greater but 30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% by weight or greater but 25% by weight or less of an α,β-unsaturated carboxylate ester. The ternary copolymer has excellent resilience performance. For the binary copolymers and the ternary copolymers, preferable α-olefins are ethylene and propylene, while preferable α,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid. Particularly preferable ionomer resins are a copolymer formed with ethylene and acrylic acid and a copolymer formed with ethylene and methacrylic acid.

In the binary copolymers and the ternary copolymers, some of the carboxyl groups are neutralized with metal ions. Examples of metal ions for use in neutralization include sodium ion, potassium ion, lithium ion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymium ion. The neutralization may be carried out with two or more types of metal ions. Particularly suitable metal ions in light of resilience performance and durability of the golf ball 2 are sodium ion, zinc ion, lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”, “Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan 1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “Himilan AM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan AM7337”, “Himilan MK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”, “Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn 8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”, “Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufactured by E.I. du Pont de Nemours and Company; and trade names “IOTEK 7010”, “IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK 8030”, manufactured by ExxonMobil Chemical Company.

Two or more ionomer resins may be used in combination for the inner cover 6. An ionomer resin neutralized with a monovalent metal ion, and an ionomer resin neutralized with a bivalent metal ion may be used in combination.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the inner cover 6 in an adequate amount.

From the standpoint that an outer-hard/inner-soft structure can be achieved in the sphere consisting of the core 4 and the inner cover 6, the inner cover 6 preferably has a hardness Hi greater than the surface hardness Hs of the core 4. In light of suppression of spin, the difference (Hi−Hs) therebetween is preferably equal to or greater than 2, more preferably equal to or greater than 4, and particularly preferably equal to or greater than 6.

The hardness Hi is measured with a JIS-C type hardness scale mounted to an automated rubber hardness measurement machine (trade name “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slab that is formed by hot press and that has a thickness of about 2 mm is used. A slab kept at 23° C. for two weeks is used for the measurement. At the measurement, three slabs are stacked. A slab formed from the same resin composition as the resin composition of the inner cover 6 is used for the measurement.

From the standpoint that an outer-hard/inner-soft structure can be achieved in the sphere consisting of the core 4 and the inner cover 6, the JIS-C hardness Hi of the inner cover 6 is preferably equal to or greater than 75, more preferably equal to or greater than 80, and particularly preferably equal to or greater than 83. In light of feel at impact of the golf ball 2, the hardness Hi is preferably equal to or less than 98 and particularly preferably equal to or less than 97.

The hardness Hi of the inner cover 6 may be less than the surface hardness Hs of the core 4. In this case, the golf ball has excellent feel at impact.

The inner cover 6 preferably has a thickness Ti of 0.5 mm or greater but 1.6 mm or less. In the sphere that includes the inner cover 6 having a thickness Ti of 0.5 mm or greater, the spin suppression effect provided by the outer-hard/inner-soft structure is great. In this respect, the thickness Ti is particularly preferably equal to or greater than 0.7 ram. The golf ball 2 that includes the inner cover 6 having a thickness Ti of 1.6 mm or less can include a large core 4. The large core 4 can contribute to the resilience performance of the golf ball 2. In this respect, the thickness Ti is particularly preferably equal to or less than 1.2 mm.

A preferable resin that can be used in combination with an ionomer resin in the resin composition of the inner cover 6 is a styrene block-containing thermoplastic elastomer. The styrene block-containing thermoplastic elastomer has excellent compatibility with ionomer resins. A resin composition including the styrene block-containing thermoplastic elastomer has excellent fluidity.

The styrene block-containing thermoplastic elastomer includes a polystyrene block as a hard segment, and a soft segment. A typical soft segment is a diene block. Examples of compounds for the diene block include butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two or more compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenated SBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenated SBS include styrene-ethylene-butylene-styrene block copolymers (SEBS). Examples of hydrogenated SIS include styrene-ethylene-propylene-styrene block copolymers (SEPS). Examples of hydrogenated SIBS include styrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content of the styrene component in the styrene block-containing thermoplastic elastomer is preferably equal to or greater than 10% by weight, more preferably equal to or greater than 12% by weight, and particularly preferably equal to or greater than 15% by weight. In light of feel at impact of the golf ball 2, the content is preferably equal to or less than 50% by weight, more preferably equal to or less than 47% by weight, and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplastic elastomers include an alloy of an olefin and one or more members selected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, and SEEPS. The olefin component in the alloy is presumed to contribute to improvement of compatibility with ionomer resins. Use of this alloy improves the resilience performance of the golf ball 2. An olefin having 2 to 10 carbon atoms is preferably used. Examples of suitable olefins include ethylene, propylene, butene, and pentene. Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “Rabalon T3221C”, “Rabalon T3339C”, “Rabalon SJ4400N”, “Rabalon SJ5400N”, “Rabalon SJ6400N”, “Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N”, and “Rabalon SR04”, manufactured by Mitsubishi Chemical Corporation. Other specific examples of styrene block-containing thermoplastic elastomers include trade name “Epofriend A1010” manufactured by Daicel Chemical Industries, Ltd., and trade name “Septon HG-252” manufactured by Kuraray Co., Ltd.

For the outer cover 8, a resin composition is suitably used. Examples of the base polymer of the resin composition include ionomer resins, polystyrenes, polyesters, polyamides, and polyolefins.

Particularly preferable base polymers are ionomer resins. The golf ball 2 that includes the outer cover 8 including an ionomer resin has excellent resilience performance. An ionomer resin and another resin may be used in combination for the outer cover 8. In this case, the principal component of the base polymer is preferably the ionomer resin. Specifically, the proportion of the ionomer resin to the entire base polymer is preferably equal to or greater than 50% by weight, more preferably equal to or greater than 60% by weight, and particularly preferably equal to or greater than 70% by weight. The outer cover 8 can include the ionomer resin described above for the inner cover 6.

A preferable resin that can be used in combination with an ionomer resin is an ethylene-(meth)acrylic acid copolymer. The copolymer is obtained by a copolymerization reaction of a monomer composition that includes ethylene and (meth)acrylic acid. In the copolymer, some of the carboxyl groups are neutralized with metal ions. The copolymer includes 3% by weight or greater but 25% by weight or less of a (meth)acrylic acid component. An ethylene-(meth)acrylic acid copolymer having a polar functional group is particularly preferred. A specific example of the ethylene-(meth)acrylic acid copolymer is trade name “NUCREL” manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

According to need, a coloring agent such as titanium dioxide, a filler such as barium sulfate, a dispersant, an antioxidant, an ultraviolet absorber, a light stabilizer, a fluorescent material, a fluorescent brightener, and the like are included in the resin composition of the outer cover 8 in an adequate amount.

In light of flight performance upon a shot with a fairway wood, the outer cover 8 has a JIS-C hardness Ho of preferably 80 or greater, more preferably 85 or greater, and particularly preferably 88 or greater. In light of feel at impact, the hardness Ho is preferably equal to or less than 98, more preferably equal to or less than 96, and particularly preferably equal to or less than 94. The hardness Ho is measured by the same measurement method as that for the hardness Hi.

In light of flight performance, the hardness Ho of the outer cover 8 is preferably greater than the hardness Hs at the surface of the core 4. The difference (Ho−Hs) therebetween is preferably equal to or greater than 5 and particularly preferably equal to or greater than 7. The difference (Ho−Hs) is preferably equal to or less than 20.

In light of flight performance, the hardness Ho of the outer cover 8 is preferably greater than the hardness Hi of the inner cover 6. The difference (Ho−Hi) is preferably equal to or greater than 3 and particularly preferably equal to or greater than 5. The difference (Ho−Hi) therebetween is preferably equal to or less than 20.

In the golf ball 2, a component having the greatest hardness in a range from the central point to the surface is the outer cover 8. The outer cover 8 can contribute to flight performance upon a shot with a fairway wood.

For forming the outer cover 8, known methods such as injection molding, compression molding, and the like can be used. When forming the outer cover 8, the dimples 14 are formed by pimples formed on the cavity face of a mold.

In light of feel at impact, the sum (Ti+To) of the thickness Ti of the inner cover 6 and the thickness To of the outer cover 8 is preferably equal to or less than 2.5 mm, more preferably equal to or less than 2.3 mm, and particularly preferably equal to or less than 2.1 mm. In light of durability of the golf ball 2, the sum (Ti+To) is preferably equal to or greater than 0.3 mm, more preferably equal to or greater than 0.5 mm, and particularly preferably equal to or greater than 0.8 mm.

In light of feel at impact, the golf ball 2 has an amount of compressive deformation (comp'n) of preferably 2.5 mm or greater, more preferably 2.8 mm or greater, and particularly preferably 3.0 mm or greater. In light of resilience performance, the amount of compressive deformation is preferably equal to or less than 4.0 mm, more preferably equal to or less than 3.6 mm, and particularly preferably equal to or less than 3.4 mm.

For measurement of the amount of compressive deformation, a YAMADA type compression tester is used. In the tester, a sphere such as the core 4, the golf ball 2, or the like is placed on a hard plate made of metal. Next, a cylinder made of metal gradually descends toward the sphere. The sphere, squeezed between the bottom face of the cylinder and the hard plate, becomes deformed. A migration distance of the cylinder, starting from the state in which an initial load of 98 N is applied to the sphere up to the state in which a final load of 1274 N is applied thereto, is measured.

The golf ball may include a center formed from a rubber composition that includes the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that does not include the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that does not include the acid and/or the salt (d). The rubber composition of the center is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the center is appropriate.

The golf ball may include a center formed from a rubber composition that does not include the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that includes the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that does not include the acid and/or the salt (d). The rubber composition of the inner envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the inner envelope layer is appropriate.

The golf ball may include a center formed from a rubber composition that does not include the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that does not include the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that includes the acid and/or the salt (d). The rubber composition of the outer envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the outer envelope layer is appropriate.

The golf ball may include a center formed from a rubber composition that includes the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that includes the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that does not include the acid and/or the salt (d). The rubber composition of the center is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the center is appropriate. The rubber composition of the inner envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the inner envelope layer is appropriate.

The golf ball may include a center formed from a rubber composition that includes the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that does not include the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that includes the acid and/or the salt (d). The rubber composition of the center is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the center is appropriate. The rubber composition of the outer envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the outer envelope layer is appropriate.

The golf ball may include a center formed from a rubber composition that does not include the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that includes the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that includes the acid and/or the salt (d). The rubber composition of the inner envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the inner envelope layer is appropriate. The rubber composition of the outer envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the outer envelope layer is appropriate.

The golf ball may include a center formed from a rubber composition that includes the acid and/or the salt (d), an inner envelope layer formed from a rubber composition that includes the acid and/or the salt (d), and an outer envelope layer formed from a rubber composition that includes the acid and/or the salt (d). The rubber composition of the center is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the center is appropriate. The rubber composition of the inner envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the inner envelope layer is appropriate. The rubber composition of the outer envelope layer is the same as the rubber composition of the inner envelope layer 12 shown in FIG. 1. A hardness distribution of the outer envelope layer is appropriate.

EXAMPLES Example 1

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (trade name “BR-730”, manufactured by JSR Corporation), 28 parts by weight of methacrylic acid, 34 parts by weight of magnesium oxide, and 0.75 parts by weight of dicumyl peroxide. This rubber composition was placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 170° C. for 25 minutes to obtain a center with a diameter of 15 mm.

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (the aforementioned “BR-730”), 25.5 parts by weight of zinc diacrylate (trade name “Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5.0 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-thionaphthol, and 0.75 parts by weight of dicumyl peroxide. Half shells were formed from this rubber composition. The center was covered with two of these half shells. The center and the half shells were placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 150° C. for 20 minutes to obtain a sphere with a diameter of 30.0 mm. An inner envelope layer was formed from the rubber composition.

A rubber composition was obtained by kneading 100 parts by weight of a high-cis polybutadiene (the aforementioned “BR-730”), 26.0 parts by weight of zinc diacrylate (the aforementioned “Sanceler SR”), 5.0 parts by weight of zinc oxide, an appropriate amount of barium sulfate, 0.2 parts by weight of 2-thionaphthol, 10.0 parts by weight of zinc stearate, and 0.75 parts by weight of dicumyl peroxide. Half shells were formed from this rubber composition. The sphere consisting of the center and the inner envelope layer was covered with two of these half shells. The sphere and the half shells were placed into a mold including upper and lower mold halves each having a hemispherical cavity, and heated at 150° C. for 20 minutes to obtain a core with a diameter of 39.1 mm. An outer envelope layer was formed from the rubber composition. The amount of barium sulfate was adjusted such that the specific gravity of the outer envelope layer is equal to the specific gravity of each of the center and the inner envelope layer, and the weight of a ball is 45.4 g.

A resin composition was obtained by kneading 40 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weight of another ionomer resin (the aforementioned “Himilan AM7329”), 20 parts by weight of a styrene block-containing thermoplastic elastomer (the aforementioned “Rabalon T3221C”), and 6 parts by weight of titanium dioxide with a twin-screw kneading extruder. The core was placed into a mold including upper and lower mold halves each having a hemispherical cavity. The resin composition was injected around the core by injection molding to form an inner cover with a thickness of 1.0 mm.

A resin composition was obtained by kneading 5 parts by weight of an ionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weight of another ionomer resin (the aforementioned “Himilan 1555”), 55 parts by weight of still another ionomer resin (the aforementioned “Himilan AM7329), 30 parts by weight of an ethylene-acrylic acid copolymer (trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2 parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”, manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder. The sphere consisting of the core and the inner cover was placed into a final mold having a large number of pimples on its cavity face. The resin composition was injected around the sphere by injection molding to form an outer cover with a thickness of 0.8 mm. Dimples having a shape that is the inverted shape of the pimples were formed on the outer cover. A clear paint including a two-component curing type polyurethane as a base material was applied to this outer cover to obtain a golf ball of Example 1 with a diameter of 42.7 mm.

Examples 2 to 18 and Comparative Examples 1 to 7

Golf balls of Examples 2 to 18 and Comparative Examples 1 to 7 were obtained in the same manner as Example 1, except the specifications of the center, the inner envelope layer, the outer envelope layer, the inner cover, and the outer cover were as shown in Tables 11 to 15 below. The composition of the center is shown in detail in Table 1 below. The compositions of the inner envelope layer and the outer envelope layer are shown in detail in Tables 2 to 3 below. The compositions of the inner cover and the outer cover are shown in detail in Tables 4 to 5 below. A hardness of the core is shown in Tables 6 to 10 below. The golf balls of Comparative Examples 1 and 2 do not have an inner envelope layer. The golf balls of Example 4 and Comparative Examples 1, 2, and 6 do not have an inner cover.

[Hit with Fairway Wood (W#5)]

A fairway wood with a titanium head (trade name “XXIO”, manufactured by DUNLOP SPORTS CO. LTD.) was attached to a swing machine manufactured by True Temper Co. A golf ball was hit under the condition of a head speed of 37 m/sec. The spin rate was measured immediately after the hit. Furthermore, the distance from the launch point to the stop point was measured. The average value of data obtained by 10 measurements is shown in Tables 11 to 15 below.

[Feel at Impact]

Ten golf players hit golf balls with fairway woods (w#5), and were asked about feel at impact. The evaluation was categorized as follows based on the number of golf players who answered, “the feel at impact was favorable”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in Tables 11 to 15 below.

TABLE 1 Composition of Center (parts by weight) A B BR-730 100 100 Methacrylic acid 28 — Magnesium oxide 34 — Sanceler SR — 20 Zinc oxide — 5 Barium sulfate — * 2-thionaphthol — 0.2 Zinc octoate — 5.0 Dicumyl peroxide 0.75 0.75 * Appropriate amount

TABLE 2 Composition of Envelope Layer (parts by weight) 1 2 3 4 5 6 BR-730 100 100 100 100 100 100 Sanceler SR 26.0 27.5 29.5 31.5 27.0 26.5 Zinc oxide 5 5 5 5 5 5 Barium sulfate * * * * * * 2-thionaphthol 0.2 0.2 0.2 0.2 0.2 0.2 Zinc octoate — — — — — — Zinc laurate — — — — — — Zinc myristate — — — — — — Zinc stearate 10.0 20.0 30.0 40.0 — 0.5 Dicumyl 0.75 0.75 0.75 0.75 0.75 0.75 peroxide * Appropriate amount

TABLE 3 Composition of Envelope Layer (parts by weight) 7 8 9 10 11 12 BR-730 100 100 100 100 100 100 Sanceler SR 25.5 25.0 25.5 26.0 25.5 25.5 Zinc oxide 5 5 5 5 5 5 Barium sulfate * * * * * * 2-thionaphthol 0.2 0.2 0.2 0.2 0.2 0.2 Zinc octoate 2.5 5.0 — — — — Zinc laurate — — 10.0 — — — Zinc myristate — — — 5.0 10.0 — Zinc stearate — — — — — — Dicumyl 0.75 0.75 0.75 0.75 0.75 0.75 peroxide * Appropriate amount

The details of the compounds listed in Tables 1 to 3 are as follows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation (cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% by weight, Mooney viscosity (ML₁₊₄(100° C.)): 55, molecular weight distribution (Mw/Mn): 3)

Methacrylic acid: a product of Mitsubishi Rayon Co., Ltd. Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD. (a product coated with 10% by weight of stearic acid)

Magnesium oxide: trade name “Magsarat 150ST” manufactured by Kyowa Chemical Industry Co., Ltd.

2-thionaphthol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Dicumyl peroxide: a product of NOF Corporation

TABLE 4 Composition of Cover (parts by weight) C1 C3 C4 C5 Himilan AM7337  5 30 — 51 Himilan 1555 10 — — — Himilan 1605 — — 50 — Himilan AM7329 55 30 50 40 NUCREL N1050H 30 — — — Rabalon T3221C — 40 —  9 Titanium dioxide (A220)  3  6  4  6 TINUVIN 770   0.2   0.2 — — Hardness (JIS-C) 92 71 96 89 Hardness (Shore D) 61 40 65 58

TABLE 5 Composition of Cover (parts by weight) C6 C7 C8 C9 Himilan AM7337 40 24 26 26 Himilan 1555 — — — — Himilan 1605 — — — — Himilan AM7329 40 50 40 26 NUCREL N1050H — — — — Rabalon T3221C 20 26 34 48 Titanium dioxide (A220)  6  6  6  6 TINUVIN 770 — — — — Hardness (JIS-C) 85 83 76 65 Hardness (Shore D) 54 52 45 35

TABLE 6 Hardness (JIS-C) Comp. Ex. Comp. Ex. Ex. Ex. 1 1 Ex. 2 2 3 Composition Center A A B B A Inner envelope layer — 12   — 5  1  Outer envelope layer 1  1  5  12   12   H(0.0) 55.0 55.0 55.0 55.0 55.0 H(2.5) 56.0 56.0 58.5 58.5 56.0 H(5.0) 58.0 58.0 60.5 60.5 58.0 H(7.0) 60.0 60.0 — — 60.0 H(7.5) — — 62.5 62.5 — H(8.0) 64.6 65.2 — — 64.6 H(9.5) — — 65.0 65.0 — H(10.0) 67.0 67.9 — — 67.0 H(10.5) — — 69.1 69.1 — H(12.5) 71.8 71.4 70.6 70.6 71.8 H(14.5) — 74.5 — 73.5 75.0 H(15.0) 76.0 — 74.1 — — H(15.5) — 77.0 — 77.0 77.0 H(17.5) 79.5 79.5 79.0 78.0 78.0 Hs 83.0 83.0 83.0 81.0 81.0 Hs − H(0.0) 28.0 28.0 28.0 26.0 26.0

TABLE 7 Hardness (JIS-C) Ex. Ex. Ex. Ex. Ex. 4 5 6 7 8 Composition Center A A A A A Inner envelope layer 1  1  1  1  1  Outer envelope layer 12   12   12   12   12   H(0.0) 55.0 55.0 55.0 55.0 55.0 H(2.5) 56.0 56.0 56.0 56.0 56.0 H(5.0) 58.0 58.0 58.0 58.0 58.0 H(7.0) 60.0 60.0 60.0 60.0 60.0 H(7.5) — — — — — H(8.0) 64.6 64.6 64.6 64.6 64.6 H(9.5) — — — — — H(10.0) 67.0 67.0 67.0 67.0 67.0 H(10.5) — — — — — H(12.5) 71.8 71.8 71.8 71.8 71.8 H(14.5) 75.0 75.0 75.0 75.0 75.0 H(15.0) — — — — — H(15.5) 77.0 77.0 77.0 77.0 77.0 H(17.5) 78.0 78.0 78.0 78.0 78.0 Hs 81.0 81.0 81.0 81.0 81.0 Hs − H(0.0) 26.0 26.0 26.0 26.0 26.0

TABLE 8 Hardness (JIS-C) Ex. Comp Comp Comp Ex. 9 Ex. 3 Ex. 4 Ex. 5 10 Composition Center A A A A A Inner envelope layer 1  1  1  1  2  Outer envelope layer 12   12   12   12   12   H(0.0) 55.0 55.0 55.0 55.0 55.0 H(2.5) 56.0 56.0 56.0 56.0 56.0 H(5.0) 58.0 58.0 58.0 58.0 58.0 H(7.0) 60.0 60.0 60.0 60.0 60.0 H(7.5) — — — — — H(8.0) 64.6 64.6 64.6 64.6 62.8 H(9.5) — — — — — H(10.0) 67.0 67.0 67.0 67.0 66.6 H(10.5) — — — — — H(12.5) 71.8 71.8 71.8 71.8 73.7 H(14.5) 75.0 75.0 75.0 75.0 75.4 H(15.0) — — — — — H(15.5) 77.0 77.0 77.0 77.0 77.0 H(17.5) 78.0 78.0 78.0 78.0 78.0 Hs 81.0 81.0 81.0 81.0 81.0 Hs − H(0.0) 26.0 26.0 26.0 26.0 26.0

TABLE 9 Hardness (JIS-C) Ex. Ex. Ex. Comp. Ex. 11 12 13 Ex. 6 14 Composition Center A A A A A Inner envelope layer 3  4  6  5  7  Outer envelope layer 12   12   12   12   12   H(0.0) 55.0 55.0 55.0 55.0 55.0 H(2.5) 56.0 56.0 56.0 56.0 56.0 H(5.0) 58.0 58.0 58.0 58.0 58.0 H(7.0) 60.0 60.0 60.0 60.0 60.0 H(7.5) — — — — — H(8.0) 64.0 64.3 68.8 67.7 65.2 H(9.5) — — — — — H(10.0) 66.8 67.0 70.0 68.6 67.4 H(10.5) — — — — — H(12.5) 71.0 70.4 71.2 70.6 71.0 H(14.5) 72.1 70.5 74.8 74.1 75.3 H(15.0) — — — — — H(15.5) 77.0 77.0 77.0 77.0 77.0 H(17.5) 78.0 78.0 78.0 78.0 78.0 Hs 81.0 81.0 81.0 81.0 81.0 Hs − H(0.0) 26.0 26.0 26.0 26.0 26.0

TABLE 10 Hardness (JIS-C) Ex. Ex. Ex. Ex. Comp. 15 16 17 18 Ex. 7 Composition Center A A A A A Inner envelope layer 8  9  10   11   5  Outer envelope layer 12   12   12   12   12   H(0.0) 55.0 55.0 55.0 55.0 55.0 H(2.5) 56.0 56.0 56.0 56.0 56.0 H(5.0) 58.0 58.0 58.0 58.0 58.0 H(7.0) 60.0 60.0 60.0 60.0 60.0 H(7.5) — — — — — H(8.0) 63.8 64.4 67.0 65.5 67.7 H(9.5) — — — — — H(10.0) 67.9 66.4 68.5 67.4 68.6 H(10.5) — — — — — H(12.5) 73.8 71.0 70.1 71.8 70.6 H(14.5) 77.8 77.0 76.7 77.5 74.1 H(15.0) — — — — — H(15.5) 77.0 77.0 77.0 77.0 77.0 H(17.5) 78.0 78.0 78.0 78.0 78.0 Hs 81.0 81.0 81.0 81.0 81.0 Hs − H(0.0) 26.0 26.0 26.0 26.0 26.0

TABLE 11 Results of Evaluation Comp. Ex. Comp. Ex. Ex. Ex. 1 1 Ex. 2 2 3 Center Acid/salt (PHR) 0.0 0.0 5.0 5.0 0.0 Diameter (mm) 15.00 15.00 20.00 20.00 15.00 Inner envelope layer Acid/salt (PHR) — 0.0 — 0.0 10.0 Thickness (mm) — 7.50 — 5.00 7.50 Outer envelope layer Acid/salt (PHR) 10.0 10.0 0.0 0.0 0.0 Thickness (mm) 12.05 4.55 9.55 4.55 4.55 Core Diameter (mm) 39.10 39.10 39.10 39.10 39.10 Comp'n (mm) 3.90 3.91 3.93 3.89 3.86 Inner cover Composition — C6 — C6 C6 Thickness Ti (mm) — 1.00 _(—) 1.00 1.00 Hardness Hi (JIS-C) — 85 — 85 85 Outer cover Composition C1 C1 C1 C1 C1 Thickness To (mm) 1.80 0.80 1.80 0.80 0.80 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Comp'n (mm) 3.23 3.23 3.23 3.23 3.18 Atti 74.6 74.6 74.6 74.6 76.3 Spin (rpm) 3865 3780 3860 3835 3760 Flight distance (m) 179.8 181.1 179.9 180.5 181.6 Feel D B C B B

TABLE 12 Results of Evaluation Ex. Ex. Ex. Ex. Ex. 4 5 6 7 8 Center Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Diameter (mm) 15.00 15.00 15.00 15.00 15.00 Inner envelope layer Acid/salt (PHR) 10.0 10.0 10.0 10.0 10.0 Thickness (mm) 7.50 7.50 7.50 7.50 7.50 Outer envelope layer Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Thickness (mm) 4.55 4.55 4.25 3.95 4.55 Core Diameter (mm) 39.10 39.10 38.50 37.90 39.10 Comp'n (mm) 3.86 3.86 3.86 3.86 3.86 Inner cover Composition — C5 C6 C6 C9 Thickness Ti (mm) — 1.00 1.00 1.00 1.00 Hardness Hi (JIS-C) — 89 85 85 65 Outer cover Composition C1 C1 C1 C1 C1 Thickness To (mm) 1.80 0.80 1.10 1.40 0.80 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Comp'n (mm) 3.16 3.23 3.18 3.16 3.26 Atti 76.9 74.6 76.3 76.9 73.6 Spin (rpm) 3775 3740 3790 3770 3840 Flight distance (m) 181.2 181.9 181.0 181.4 180.4 Feel B B B B A

TABLE 13 Results of Evaluation Ex. Comp. Comp. Comp. Ex. 9 Ex. 3 Ex. 4 Ex. 5 10 Center Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Diameter (mm) 15.00 15.00 15.00 15.00 15.00 Inner envelope layer Acid/salt (PHR) 10.0 10.0 10.0 10.0 20.0 Thickness (mm) 7.50 7.50 7.50 7.50 7.50 Outer envelope layer Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Thickness (mm) 4.55 4.55 4.55 4.55 4.55 Core Diameter (mm) 39.10 39.10 39.10 39.10 39.10 Comp'n (mm) 3.86 3.86 3.86 3.86 3.87 Inner cover Composition C8 C8 C4 C5 C6 Thickness Ti (mm) 1.00 1.00 1.00 1.00 1.00 Hardness Hi (JIS-C) 76 76 96 89 85 Outer cover Composition C1 C3 C1 C3 C1 Thickness To (mm) 0.80 0.80 0.80 0.80 0.80 Hardness Ho (JIS-C) 92 71 92 71 92 Ball Comp'n (mm) 3.23 3.31 3.13 3.23 3.21 Atti 74.6 71.9 77.9 74.6 75.2 Spin (rpm) 3830 4070 3750 3965 3765 Flight distance (m) 180.6 178.3 181.7 178.9 181.5 Feel A B D C B

TABLE 14 Results of Evaluation Ex. Ex. Ex. Comp. Ex. 11 12 13 Ex. 6 14 Center Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Diameter (mm) 15.00 15.00 15.00 15.00 15.00 Inner envelope layer Acid/salt (PHR) 30.0 40.0 0.5 0.0 2.5 Thickness (mm) 7.50 7.50 7.50 7.50 7.50 Outer envelope layer Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Thickness (mm) 4.55 4.55 4.55 4.55 4.55 Core Diameter (mm) 39.10 39.10 39.10 39.10 39.10 Comp'n (mm) 3.86 3.87 3.87 3.91 3.88 Inner cover Composition C6 C6 C7 — C7 Thickness Ti (mm) 1.00 1.00 1.00 — 1.00 Hardness Hi (JIS-C) 85 85 83 — 83 Outer cover Composition C1 C1 C1 C1 C1 Thickness To (mm) 0.80 0.80 0.80 1.80 0.80 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Comp'n (mm) 3.2 3.21 3.24 3.21 3.25 Atti 75.6 75.2 74.2 75.2 73.9 Spin (rpm) 3785 3845 3850 3905 3800 Flight distance (m) 181.1 180.3 180.2 179.5 180.9 Feel B B B C B

TABLE 15 Results of Evaluation Ex. Ex. Ex. Ex. Comp. 15 16 17 18 Ex. 7 Center Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Diameter (mm) 15.00 15.00 15.00 15.00 15.00 Inner envelope layer Acid/salt (PHR) 5.0 10.0 5.0 10.0 0.0 Thickness (mm) 7.50 7.50 7.50 7.50 7.50 Outer envelope layer Acid/salt (PHR) 0.0 0.0 0.0 0.0 0.0 Thickness (mm) 4.55 4.55 4.55 4.55 4.55 Core Diameter (mm) 39.10 39.10 39.10 39.10 39.10 Comp'n (mm) 3.84 3.86 3.87 3.85 3.91 Inner cover Composition C7 C7 C7 C7 C6 Thickness Ti (mm) 1.00 1.00 1.00 1.00 1.00 Hardness Hi (JIS-C) 83 83 83 83 83 Outer cover Composition C1 C1 C1 C1 C1 Thickness To (mm) 0.80 0.80 0.80 0.80 0.80 Hardness Ho (JIS-C) 92 92 92 92 92 Ball Comp'n (mm) 3.21 3.23 3.24 3.22 3.28 Atti 75.2 74.6 74.2 74.9 72.9 Spin (rpm) 3775 3805 3815 3795 3855 Flight distance (m) 181.3 180.8 180.7 181.0 180.0 Feel B B B B B

As shown in Tables 11 to 15, the golf balls according to Examples are excellent in various performance characteristics. From the results of evaluation, advantages of the present invention are clear.

The golf ball according to the present invention can be used for playing golf on golf courses and practicing at driving ranges. The above descriptions are merely for illustrative examples, and various modifications can be made without departing from the principles of the present invention. 

What is claimed is:
 1. A golf ball comprising a core and a cover positioned outside the core, wherein the core comprises a center, an inner envelope layer positioned outside the center, and an outer envelope layer positioned outside the inner envelope layer, the cover comprises one or more layers, at least one of the center, the inner envelope layer, and the outer envelope layer is formed from a rubber composition that includes: (a) a base rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator; and (d) an acid and/or a salt, the co-crosslinking agent (b) is: (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms, and a component having the greatest hardness in a range from a central point of the golf ball to a surface of the golf ball is an outermost layer of the cover.
 2. The golf ball according to claim 1, wherein the cover comprises an inner cover and an outer cover positioned outside the inner cover.
 3. The golf ball according to claim 2, wherein a difference (Ho−Hs) between a JIS-C hardness Ho of the outer cover and a JIS-C hardness Hs at a surface of the core is equal to or greater than
 5. 4. The golf ball according to claim 2, wherein a difference (Ho−Hi) between a JIS-C hardness Ho of the outer cover and a JIS-C hardness Hi of the inner cover is equal to or greater than
 3. 5. The golf ball according to claim 1, wherein an amount of the acid and/or the salt (d) is equal to or greater than 1.0 parts by weight but less than 40 parts by weight, per 100 parts by weight of the base rubber (a).
 6. The golf ball according to claim 1, wherein the acid and/or the salt (d) is a carboxylic acid and/or a salt thereof (d1).
 7. The golf ball according to claim 6, wherein a carbon number of a carboxylic acid component of the carboxylic acid and/or the salt thereof (d1) is equal to or greater than 1 but equal to or less than
 30. 8. The golf ball according to claim 6, wherein the carboxylic acid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.
 9. The golf ball according to claim 6, wherein the carboxylic acid and/or the salt thereof (d1) is a zinc salt of a carboxylic acid.
 10. The golf ball according to claim 9, wherein the zinc salt of the carboxylic acid is one or more members selected from the group consisting of zinc octoate, zinc laurate, zinc myristate, and zinc stearate.
 11. The golf ball according to claim 1, wherein the rubber composition includes 15 parts by weight or greater but 50 parts by weight or less of the co-crosslinking agent (b) per 100 parts by weight of the base rubber (a).
 12. The golf ball according to claim 1, wherein the rubber composition includes 0.2 parts by weight or greater but 5.0 parts by weight or less of the crosslinking initiator (c) per 100 parts by weight of the base rubber (a).
 13. The golf ball according to claim 1, wherein a difference (Hs−H(0.0)) between a JIS-C hardness Hs at a surface of the core and a JIS-C hardness H(0.0) of a central point of the core is equal to or greater than
 15. 14. The golf ball according to claim 2, wherein a sum (Ti+To) of a thickness Ti of the inner cover and a thickness To of the outer cover is equal to or less than 2.5 mm.
 15. The golf ball according to claim 1, wherein the rubber composition further includes an organic sulfur compound (e).
 16. The golf ball according to claim 15, wherein the organic sulfur compound (e) is at least one member selected from the group consisting of thiophenols, diphenyl disulfides, thionaphthols, thiuram disulfides, and metal salts thereof.
 17. The golf ball according to claim 15, wherein the rubber composition includes 0.05 parts by weight or greater but 5 parts by weight or less of the organic sulfur compound (e) per 100 parts by weight of the base rubber (a).
 18. The golf ball according to claim 1, wherein the rubber composition includes the α,β-unsaturated carboxylic acid (b1), and the rubber composition further includes a metal compound (f). 